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The Methods For Detecting Color Blindness, With Special Reference To The Examination Of Railroad Employes

The Methods For Detecting Color Blindness, With Special Reference To The Examination Of Railroad Employes

MANY PERSONS suppose that all that is required to test the color sense of railroad employes is to display the flags and lanterns used as signals and demand the name of the color exposed.
The experienced observer knows, however, that many color blind subjects can name colors correctly; hence, any test to be effectual must ascertain, not whether the employe can name colors correctly, but how he sees them, and whether he can safely be trusted to distinguish between the various signals on all occasions. We determine this, first, by making him pick out and place together those colors which appear to him to be the same, and second, by having him recognize colors at a distance under various degrees of illumination, thus simulating, as far as possible, the various atmospheric conditions under which railway signals may present themselves.

Holmgren's Method. The set of worsteds consists of three large test skeins: 1) light pure green; (2) rose (purple) ; (3) red, and of about one hundred and fifty small skeins of the following colors: red, orange, yellow, yellow green, pure green, bluegreen, blue, violet, purple, pink, brown, gray, including several shades of each color and at least five gradations of each tint from the deepest to the lightest.

First Test. The worsteds are placed in a confused heap on a large plane surface in a good light, and the light pure green test skein laid a little to one side. The candidate is Dow requested to pick out those skeins most resembling it in color and place them by the side of the sample. The examiner must. explain that there are no two skeins exactly alike, and that an endeavor must be made to find something similar of a lighter or darker shade. The candidate is not to compare narrowly or to rummage much among the heap, but to select with his eyes, and to use his hands chiefly to change the position of the selected skeins.

A person with a normal color sense will pick out the lighter and darker shades of green rapidly and without hesitation. He may, perhaps, include in his choice a few green skeins inclining to yellow or blue; but this is no evidence of color blindness, but rather of a lack of practice with colors.

The person completely color blind, whether to red or green, will select, with or without the greens, some confusion colors grays, drabs, stone colors, fawns, pinks, or yellows.

The person incompletely color blind, or with a feeble chromatic sense, will add to the selection of greens one or more light fawns or grays; or he may pick out a skein, hesitate, add it to the greens, and then withdraw it, and so on. When confusion colors have been selected the examiner knows that the candidate is either completely or incompletely color blind. In order to determine its nature and degree a second test is employed.

Second Test. The worsteds are mixed again, and the large rose test skein is laid to one side. The candidate is requested to pick out all the lighter and darker shades of this color; if color blind he will always select deeper colors. Those subjects who by the first test were found to have a feeble chromatic sense will make no mistakes in this test. Those who are incompletely color blind will match the rose with deeper purples. The completely red blind candidate will select blue or violet, either with or without purple. The completely green blind subjects take green or gray or one alone , either with or without purple. The violet blind subjects (rare) show a strong tendency to select blue in the first test, and red and orange, either with or without purple, in the second test. As this examination has decided the character and degree of the defect, it is not necessary to resort to the third test; but as the red skein used corresponds to the danger signals, it may occasionally be of value in convincing the officials that the candidate is unfit for duty.

Third Test. The sample for this test is a skein of bright red, to be used in the same way as the green and rose. The red blind subjects select, besides the red, green and brown shades darker than the red. The green blind subjects select green and brown shades lighter than the red. Only marked cases of color blindness will show their defect with this test.

Thomson's Method. This consists of two different sets of worsteds, which are kept apart in a double box. The first set consists of a large green test skein and twenty small skeins, each marked with a bangle having a concealed number extending from one to twenty. Among these numbers the odd ones are different shades of green, while the even numbers are grays, light browns, etc. The second set consists of a large rose testskein and twenty small skeins, which are numbered from twenty one to forty. Here the odd numbers are different shades of rose, while the ten even numbers consist of blues, greens, and grays.

In testing the worsteds are taken from the green part of the box and placed upon a table in a confused mass. The candidate is requested to select ten tints to match the large green skein. When this is done and the numbers recorded on a blank, the worsted is removed and the examiner proceeds with the second set.

Author's Method. Realizing that any test which is limited to a small number of match and confusion skeins curtails the choice and makes the defect more difficult to discover, the author has endeavored to combine the good points of Holingren's and Thomson's methods. The set consists of five large test skeins: lightpure green, rose, red, blue, and yellow, and eighty four small skeins, each marked with a bangle having a concealed letter and number. The letter denotes the color, and the number (1 to 6) denotes the shade. For example, A I indicates the lightest shade of pure green; K 6, the darkest shade of brown.

The examination is conducted in a manner similar to the Helmsmen method, with the addition of the blue and yellow tests. A record of one test is made before proceeding to the next. In matching the blue skein the color blind person first takes the darkest shades of blue, and then adds the rose skeins, because be recognizes the blue in the mixture of red and blue. In matching the yellow he adds all the green skeins that have yellow in them.

Pseudo isochromatic Plates of Stilling. The remarkable facility with which the color blind distinguish colors to which they are blind is due to an acute sensitiveness to differences in tint and intensity of light. In the pseudo isochromatic plates Stilling seeks to deprive the color blind of any aid in matching colors by selecting those which appear identical not only in tint, but also in intensity of light. On a colored surface of convenient size he painted a spot of the color mistaken for it; he then toiled or modified this spot until the color blind eye could not distinguish between the spot and the surface. Stilling then constructed ten plates. each plate containing four squares filled by small, irregular colored spots, among which other spots in a confusion color, made to conform to an Arabic figure, are placed. The test plate is held in a good light and the candidate required to distinguish the tracings. An important feature of this test is that there is no inquiry as to color.

Lantern test. An ordinary switch lantern with a four inch opening should be so arranged that pieces of colored glass can be placed in front of the light. The colors to be used are standard red, yellow, pure light green, standard green, blue, and purple. The luminosity of the light can be varied by having at band pieces of white (ground), ribbed, and different thicknesses of London smoke glass. As red and green appear to the color blind as one and the same color, only lighter or darker than the other, it is easy to deceive him by changing the luminosity of the light without altering its color. This can be done by diminishing the light or by placing pieces of ground or Londonsmoke glass before the colored light. The candidate should be seated at a distance of fifteen feet from the lantern, and, according to Dr. Edridge Green, should be rejected(1) if be calls the red green or the green red under an), circumstances (2) if he calls the white light under any circumstances red or green, or vice versa ; (3) if he calls the red green, or white light black, under any circumstances.

Quantitative Estimation of the Color sense. The lantern may also be used to make a quantitative estimation of the color sense by placing in front Of the light a metallic slide with perforations ranging from one to twenty millimeters in diameter.

Having tested and recorded the average size of the opening required by the normal eye to distinguish each color at fifteen feet, the candidate is placed at this distance and is .asked to name the colors. If he recognizes them through the one millimeter opening, his color sense is normal = 1/1. If an opening of twenty millimeters is needed, his colorsense 1/20. If he fails to recognize the color through the largest opening, he is told to approach the light slowly, and if he sees it at three feet, his color sense = 1/100, etc.

Oliver's apparatus is designed to test the color sense of employes upon the railwaygrounds at a distance of 1000 feet. It consists of twenty three shallow open wooden boxes, painted dead black, containing colored bunting placed upon a horizontal beam 15 feet from the ground. Arranged above the middle of these boxes is a large revolving box with five partitions for the test colors. The pure green test color is displayed, and the candidate, employing one eye at a time, is asked to write upon a piece of paper the number of the color in the lower row (going from left to right) that to him is the nearest match to the upper color. This experiment is repeated with the other test colors. If the apparatus is to be used at night, transparent colored glass is substituted for the colored bunting

Chibret's Photometer. An examination for color blindness is not complete without making a test of the light sense (see page 154). The most accurate instrument for this purpose is Chibret's photometer.

The candidate faces the window and looks with one eye into a tube, where he sees two equally bright disks. The eve piece is now turned until be can detect a difference in the illumination of the two disks, when the light difference is indicated on the scale. A normal eye recognizes the difference within five degrees. The light minimum is measured by making one disk entirely dark, and then turning the eye piece until he perceives the disk coming from the darkness. The scale should not register more than one or two degrees (see also page 152).


Having ascertained that the color sense of an employe is defective, the surgeon must decide whether the defect is of such a nature as to warrant his discharge, or whether he can with safety be allowed to retain his position. If the person in question is an applicant for employment, even a slight defect of the color sense should be sufficient ground for rejection. If, however, we have to deal with an old employe, one who, perhaps, has discharged his duties in a satisfactory manner, justice demands that his interests be studied so far as is consistent with safety. Every case of complete redor green blindness should be dismissed. Those who are incompletely color blind, and in the first test merely confound gray with the sample color, may be retained if the visual acuity and light sense are normal.

LAWs regulating the examination of railroad employes for form and color vision have been adopted in several States, but there is no official standard established by the United States Government for such examinations as exists in Continental countries. The State undertakings in this behalf have not been uniformly successful, as witness the experiment tried some years ago in Connecticut by which scientific experts were to be appointed by the governor and paid by the railroads. This undertaking proved a failure, as the railroad officers would not submit their employes to the scrutiny of State officials, who, by adopting their own standards, could practically discharge perhaps 15 per cent. of the railroad employes, disturb the discipline, and impair the organization of the roads.

As most of the large railroad lines run through several States, to save complications which may arise out of' separate State legislation it is found more expedient for the corporations to make their own rules and regulations for examination of their employes, using their own methods and appointing their own examiners.

It is found impracticable for corporations, owing to the large force of ophthalmic surgeons it would require, to study the refraction and make the examination as scientific as, from a medical point of view, is admittedly desirable. So it is, therefore, the endeavor of the railroads to devise and establish a general system of examination for protection of the public and its property that can be put in force by a division superintendent, acting through an intelligent assistant, under the general supervision of an ophthalmic surgeon. To this supervising surgeon all information collected could be transmitted, and he would thus be enabled to decide all doubtful cases and to protect the railroad from any danger arising from incapable employes, and at the same time to save good and faithful men from being discharged from the company's service without sufficient cause.

Such a system has been perfected by Dr. William Thomson, authorized by the Pennsylvania Railroad Company since 1880, and has been adopted by other roads from time to time, until it is protecting an aggregate total mileage to day of over one hundred thousand miles. This system has been, as here indicated, subjected to the test of experience, and has proved very satisfactory.

Visual Acuity. The standards of form vision in Continental countries and also in this country vary from 20/XX in one or both eyes to 20/XX in one and 20/L in the other, in the first class that is, for employes on the head end of an engine, while in Class 11, representing the yard and train service, the range is anywhere from 20/XX in one to 20/CC in the other.

A railroad should require for its safety two standards for entrance into its service: The standard of Class 1, representing engineers, firemen, and towermen, should require 20/XX in one eye, and not less than 20/XL in the other vision taken separately without glasses.

Hyperopia over 2 D. should ensure rejection astigmatism being eliminated. This can be readily ascertained by placing a trial frame containing 2 D. spherical lens, before the patient, and if he has with these lenses full acuity of vision, the optical defect is demonstrated. This practical test saves many complications, as a man way enter the service as a young man with strong accommodation, and when he becomes a skilled engineer, at the presbyopic age, he will not have vision sufficient to reach the standard.

Periodic examinations should be made at intervals of five years, or after an injury which may in any way affect the vision, and also after serious illness and on promotion.

The standard of Class 11, representing trainmen, conductors, brakemen, switchmen, and yardman, should require 20/XX in one and not less than 20/LXXX in the other eye, with or without glasses, and the same rules regarding reexamination apply to them.

Old employes not reaching the proper standard of the class to which they belong on re examination should be corrected and required to use glasses if they be permitted in that class or transferred to other duties.

Color sense. The color sense is requisite to enable any employe to govern his actions by day or night by colored signals.

The standard should require three points:
1. Test with colored signal flags.
11. Test by comparison of colored worsteds Holmgren's, Thomson's, Williams's, or Oliver
III. Test with colored light.
1. Test with Colored Flags. The man subjected to this test should recognize four flags, one of each color, red, white, green, and blue, and, at a distance of twenty feet, tell their color and meaning. A colored flag should also be given him to match with worsteds.

11. Test by Comparison of Colored Worsteds ', Matching their Colors without Telling their Names. Here one of the recognized tests should be used Holmgren's or some modification of this test. Holmgren's test consists in testing the power of the person to match various colors which are convenientlv used in the form of colored yarns. About one hundred and fifty tints are employed in confused mixture, and three test colors viz. light green, rose purple, and red are placed in order before the person examined, who is directed to select similar colors from the mass. By this manner the examiner can judge whether selections are correct or not from the prompt or hesitating manner in which the selection is made.

Tests which are modifications of this, as, for example, Thomson's stick test, are much simpler and more expedient for use on railroads.

111. Test with Colored Light. The ordinary railway lanterns of different colors may be used.

If the employe be found defective in his color sense, he will undoubtedly be detected by these tests. He is then allowed to go before the ophthalmic expert for final examination, who may use any other confirming test he may choose.

It is to be remembered that this is not an official standard simply the requirements to operate a railroad without danger to the public and destruction to property.

WITH the development of improved methods of generating and employing the Roentgen rays speedy and accurate means have been furnished by which not only the presence of a metallic body in the eye may be determined, but also its exact position. The early employment of the new form of radiant energy in experiments on animals' eyes gave little promise of the successful application of the method in ophthalmic surgery until Charles H. Williams of Boston and C. F. Clark' of Columbus, Ohio, each reported a case of the removal of a piece of metal from the living eye which had been previously located by the rays. Shortly afterward Max J. Stern, at the Philadelphia Polyclinic, located metallic bodies in the vitreous in four cases, and demonstrated the possibility of obtaining shadows on the photographic plate of foreign bodies situated in any part of the eyeball or orbit.

Practical Application of Rays. While numerous methods have been suggested and employed for determining the exact position of the body in the eye, the writer has found the use of two metal indicators, one pointing to the center of the cornea and the other situated to the temporal side at a known distance from the first, to be simple in application and accurate in results. Two radiographs are made to give different relations of the shadows of the indicators and the body in the eyeball or orbit one with the tube horizontal or nearly so with the plane of the indicators, and the other with the tube at any distance below this plane.

The principles of this method may be understood from the drawing (Fig. 420), in which a candle flame is used to represent the x ray tube. Rays of light coming from the candle when at A, in casting shadows upon a flat surface of two ball pointed rods and an object in a sphere representing the eye, give the view as shown on the surface C.

When the source of light is carried below the horizontal plane of the two rods to B, the shadows of the indicators take the position shown on the surface D, while the relative position of the body also changes. Knowing the distance of one of the balls from the center of the cornea and the distance between the balls, the position of the metal in the eye may be readily determined, since the shadow of the body preserves at all times a fixed relation with respect to the shadows of the indicating Balls in whatever position the candle is placed.

In practice it is essential that the axis of the eyeball shall be parallel with the two indicators and with the photographic plate; that one of the indicators points to the center of the cornea and be at a known distance therefrom; and that the two indicating balls be in a perpendicular line with the plate and at a known distance from each other. Simplicity has been secured by combining the plate holder and indicators into a special apparatus which is bound to the side of the head, as shown in Fig. 421.

The determination of the position of a foreign body in the eye by the method described may be understood from the two radiographs which are reproduced in Figs. 422, 423. These were made of a man who was shot in the face by a rabbit hunter, one of the shot penetrating at a point about 3 mm. below the superior border of the orbit of the left side. No view of the fundus of the left eye was possible, owing to the denseness of the vitreous, although a slight red reflex was present in the upper portion of the eye.

In determining the position of the body in the eye two circles, 24 mm. in diameter, are drawn upon paper, one to represent a horizontal and the other a vertical section of the average adult eyeball. Upon these circles are noted the positions of the balls of the indicators when the exposures were made.

Measurements are made upon each of the radiographs of the distance that the shadow of the foreign body is above or below the shadows of the indicators, and these distances are entered above or below the spots representing the two indicating balls on the circle showing the vertical section of the eye. Lines drawn through the points of measurement from the two radiographs (C and D and E and F) indicate the plane of shadow of the foreign body at each exposure. Where the two lines cross, therefore, must be the location of the body as measured above or below the horizontal plane of the eyeball and to the temporal or nasal side.

The location of the foreign body back of the center of the cornea is determined by measuring the distance that the shadow of the body is posterior to the shadows of the two indicating balls on the radiograph made with the tube horizontal to the plane of the indicators, marking off the measurement perpendicular to each of the spots representing the indicators on the horizontal section of the eye, and drawing a line through these points. Since this represents the plane of shadow of the foreign body when the radiograph was made, the metal must be situated at some point on this line. The location of the body as respects the vertical section of the eyeball having been determined, where a line drawn perpendicular to this position intersects the plane of shadow on the horizontal section is the situation of the body back of the anterior portion of the eyeball. When the distance of the platinum plate of the tube from the center indicating ball is known, the plane of shadow of the body on the horizontal section of the eye is determined by drawing a line directly from a point representing the tube to the spot of measurement of the shadow of the body back of the external indicator. In order to render the various measurements clear, outline drawings of the two radiographs, reduced one third in size, are shown in Figs. 425 and 426, the lettering corresponding to that employed on the diagrammatic circles.

When the photographic plate is in place at the side of the bead, it is necessary in the majority of cases to arrange the point of fixation so that the cornea is rotated slightly inward, in order that the visual axis shall be parallel with the plane of the photographic plate. This rotation of the eyeball in DO way affects the method of locating bodies within the globe, but when the body has penetrated into the orbit outside of the eyeball, the convergence necessary to ensure parallelism of the visual axis and the plate leads to error in the determination of the position of the metal. To eliminate this factor of error necessitates a knowledge of the angle of the orbit with the plate, or, what is equivalent, the amount of deviation of the eyeball from the primary position, and the employment of this angle in plotting the diagrammatic circles representing the eyeball. Another error arises from the false projection of the shadow of the body in the orbit in relation to the shadows of the indicators, Owing to greater divergence of the rays as the distance between the center indicator and the foreign body increases. This false projection may be allowed for by noting the distance of the platinum plate of the tube from the center indicator, and employing this measurement in determining the plane of shadow of the body on the horizontal section of the eye.

In making the exposures the plate is to the side of the head corresponding to the injured eye, and the tube is placed about 12 inches to the opposite side and slightly forward. The patient should lie upon his back, as this position ensures greater steadiness of the head and body than when sitting upright with :some form of head rest. An exposure of four minutes is ample to secure clear shadows of bodies in the eyeball or orbit, and with efficient apparatus good radiographs may be secured in one half this time. As the best results are achieved when the tube is run at a high vacuum, a tube should be selected which may be so operated that the resistance offered to the passage of the current does not reach a point to interfere with the generation of the rays. The cathodal terminal should be ground and polished, so that the rays are focussed to a small point upon the platinum, which ensures sharper outlines of the foreign body than when the focus point is large.

Accuracy of the Method. It has been conclusively shown in actual work that the x rays may be relied upon to determine in nearly every instance the presence or absence of a foreign body in the eye. The difficulties of shadowing the body on the plate increase with the smallness of the object, especially if it is situated to the nasal side of the eye and therefore some distance from the sensitive surface. It is evident that cases may occasionally be seen where the body is so small as to fail to cast a shadow of sufficient distinctness to be recognized in comparison with the shadows of the orbital bones, although chips of metal which are too minute to be shown by the rays seldom strike the eye with sufficient force to overcome the resistance of the ocular structures and penetrate deeply into the globe. In cases of doubt as to the presence of a metal body in the eye, several exposures should be made with the tube in various positions, in order to cause the body, if present, to be shadowed through the thinnest portion of' the orbital bones, and thereby exhibit sufficient contrast to assist in revealing its presence.

Dangers. The introduction of more powerful apparatus for the generation of the rays has reduced to a minimum the dangers of severe injury of the superficial structures of the body by decreasing the time of exposure. Persons of fair complexion are particularly susceptible to the action of the rays, although a slight redness of the skin is all that may be expected in any case in the short period required in making the negatives. It is a safe plan, however, to limit the exposures at one sitting to four, which at the most would subject the patient to the action of the rays for a period of sixteen minutes. In this way it is possible to note the effect on the skin, and, if additional radiographs are desired, postpone the second sitting for several days in case marked redness follows the first exposures.

Influence on Vision of Blind Eyes. The experiments made by Hansell,' by Wilkinson of the California School for the Blind, and by Hilgartner and Northrup conclusively show that the x rays have no power whatever of exciting vision or even light perception in an eye, diseased or normal, and are without beneficial effect in the treatment of diseases leading to blindness. These investigations were made upon a number of patients who were blind from dense opacities of the cornea, congenital cataract, or complete optic atrophy.

Introduction. The frequent practice of ophthalmic operations upon animals' eyes is of the greatest importance to the beginner in operative ophthalmology, as it enables him to become acquainted with the use of the various instruments to recognize the difference in the density of the tissues which have to be cut, to become thoroughly familiar with the technic of each operation, and to lose a certain amount of the timidity which is almost invariably present when beginning operative work upon the human eye.

Instruments. A set of instruments 'should be obtained and used for this purpose alone. The following are all which are required for practising most of the operations on the eyeball and muscles: An eye speculum, a pair of fixation forceps, an angular keratome, a v. Graefe cataract knife, a pair of iris forceps, a pair of iris scissors, a cystotome and Daviel's scoop, a cataract needle, a strabismus hook, a pair of strabismusscissors, a canaliculus knife, a small scalpel, and a few curved needles.

Choice of Eyes. Pigs' eyes are the most useful for practising the various operations. They more nearly resemble human eyes in size and density of tissue than do the eyes of other animals that are readily obtainable, and they can be easily fastened in the various masks. Sheeps' eyes are too large for the latter 'purpose, and bullocks' eyes, while useful for demonstrations before a large class, possess tissues which are too dense and are themselves too large for the instruments which are commonly employed in operations on the human eye to make them of practical value.

For operations upon the muscles, the orbits, and the lids it is necessary to have a bead with the eyes in their natural positions. For this purpose the bead of a young pig, about six weeks old, is perhaps the easiest obtained and answers the purpose very well. The butcher must be cautioned, however, to allow the bead to remain in scalding water for the shortest possible time preparatory to removing the bristles, or the eyes will be too shrunken to answer the purpose. Even with these precautions the corneas will be a trifle hazy, but if the eyeballs retain their firmness, this defect will not interfere with the subsequent practice of the operations.

If possible, all the operations should also be practised upon the head of a cadaver; but, unfortunately, it is difficult to obtain material of this character outside of the disecting rooms of medical schools, and even when it is at hand the eyes are often so shrunken and collapsed, and have undergone such great changes, that it is fully as satisfactory, if riot more so, to practise on the animal's eyes. To obtain correct ideas, however, of the topography of the parts practice on a cadaver as fresh and well preserved as possible is essential.

After practising for a time upon eyes placed in a mask and upon eyes in their natural positions in a pig's head, it is advisable to obtain some experience in operating upon the eyes of live animals. Dogs, cats, or rabbits may be used, the latter being perhaps the least expensive and most easily handled. The animals should be anesthetized with chloroform before operating, and at the conclusion of the operation the anesthesia should be pushed sufficiently far to produce death.

Time of Removal of Eyes from the Animal. As eyes always undergo various changes shortly after death which render them less valuable for operative work, they should be removed from the animal as soon as it is killed. It is especially important that they be removed before the animal is scalded preparatory to scraping off the bristles, otherwise the corneas will become so opaque and shrunken as to render them useless.

Method of Preserving Eyes for Operating Purposes. Fresh eyes are by far the best and most satisfactory for operative work. They impart to the hand a more natural sense of resistance of the tissues, and the corneas are much clearer than they can possibly be after preservation in any liquid. If it is impossible, however, to obtain them fresh when desired, they can be preserved for operating purposes for about one week by placing them in a I/10 of a 1 per cent. solution of formaldehyd. A stronger solution, though excellent as a preservative, hardens them too much for operative work. Should it be desired to preserve them even longer, they may be transferred to a solution of thymol (1 :5000), in which they will keep for several weeks (Andogsky). No matter whether fresh or preserved eyes are employed, the corneas will be found to be more or less dry, so that before beginning any operation they should be moistened with water.

The Operating Mask. It is customary when practising operations upon animals' eyes to place the latter in masks especially constructed for the purpose. The best of these is the Viennese mask seen in Fig. 427. This represents a human face with most of its relations preserved, and in the orbital cavities are placed removable sockets in which the animal's eyes can be firmly held. These sockets permit the eyes to be moved i n all directions, and by turning a central screw on which the eye rests the latter can be tightened or loosened, so that the intraocular tension may be decreased or diminished at will. The face is so attached to its base that it can be placed at different angles, and is made of bard rubber to prevent absorption of the various ocular fluids. Other masks known as 11 phantom faces" and made of papier mache may be also used for the same purpose.

The Home made Mask. If the student does not possess the Viennese mask or a phantom face, a fairly satisfactory substitute may be constructed at home from a small box and apiece of cork. The latter should be sufficiently thick to enable the hand to move freely without striking the lid of the box, and is glued to the latter as shown in Fig. 428. An eye is readily stout pins, and the lid can be placed at any fastened to this by means of four tacks or angle desired.

Should no mask be at band, an eye can be wrapped in a towel and held in the hand of an assistant, which rests firmly on a table while the different operations on the eyeball are being practised (Fig. 429). The greatest objection to this method is the impossibility of holding an eye firmly without making considerable pressure, which spoils, to a certain extent, most operative procedures. The method is of great value, however, in practising puncture and counter puncture and the different varieties of corneal sections, and in these the assistant may be dispensed with, the eye being held in one hand while the knife is manipulated with the other.

Preparation of the Eye for the Mask, When the eyes are removed from the pigs they have attached to them, as a rule, the stumps of the muscles, some conjunctiva, and more or less of the orbital fat. Enough of this should be trimmed off to enable the eye to fit easily into the socket of the mask, but at the same time care must be exercised not to remove too much or the eye cannot be held sufficiently tight for the satisfactory performance of an operation. A little practice will soon enable the student to know just how much tissue to remove, so that the strongest possible grasp may be maintained during the whole operation.

The shape of the pig's cornea differs somewhat from that of the human cornea, and in placing pigs' eyes in the mask socket the round end should be turned upward. In this manner the shortest diameter of the eve is horizontal, and the iris is less apt to fall in front of the knife in making corneal sections than when placed in ally other position.

Before attempting to fit an eve into the mask socket the latter should be removed from the mask and the cavity made as large as possible by means of the screw on which the eye is to rest. After this an eye is placed in position, and a small circular metal hand containing several teeth is pushed over it to hold it in place.

General Directions. Before beginning to practise any operation the instruments required for that operation should be selected and placed within easy reaching distance in the order in which they are to be used. If some one is assisting, the operator should not remove his eyes from the field of operation more than is absolutely required, the assistant placing in his bands each instrument as it is needed and removing the ones that have just been employed. The operator should also be careful to assume only such positions in relation to the animal's eye, or to the mask, as could be easily attained if operating on the human eye, and should* studiously avoid any but the proper manner of holding the instruments. In other words, as much attention should be paid to detail as if the operation were being performed on a human eye. Thus only proper habits will be formed, for the habits formed in this work will adhere to the student in his later work upon human eyes and, if they be incorrect, will be difficult to overcome.

Operations which can be Pradised. In general, most of the operations which are performed on the human eye may be practised on animals' eyes employed as previously described. Figs. 430 433' are sufficiently illustrative of some of the main operations.

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