224 EXPERIMENTAL PHYSIOLOGY 



reflected rays from any point of the fundus in a normal eye then emerge 

 parallel, and the observer's eye should also be accommodated for distant 

 vision in order that a clear image may be thrown upon his retina. As this 

 latter is at first difficult and requires some practice, it will be found easier to 

 insert a biconcave lens behind the aperture of the mirror to render the rays 

 divergent, and then the observer accommodates until distinct vision is 

 obtained. 



Fig. 175 shows the course of the rays of light when employing this 

 method, s is the source of light and m m the mirror which reflects the rays 

 of light, these are focussed by the eye being examined (e) to a point in 

 the vitreous, and from this cause a diffuse lighting of the fundus. Rays of 

 light issuing from a pointy emerge from the eye parallel to one another, and 

 entering the observing eye e 1 are brought to a focus, p 1 , which lies on the retina, 

 as the eye is accommodated for distant vision. Similarly a point m will give 

 rise to an image at m 1 , and a point n at «'. 



HOLMGREN'S METHOD OF TESTING COLOUR VISION 



In this method a large number of sample skeins of worsteds of 

 different colours and shades of colour are employed. The colours 

 include reds, oranges, yellows, yellowish greens, pure greens, blue 

 greens, blues, violets, purples, pinks, browns, and greys. The method 

 of testing consists in picking out one of the skeins and requesting the 

 subject to be tested to select skeins from the pile which resemble it in 

 colour. No two skeins are alike, so that the examinee is to pick out 

 skeins which appear of the same colour, though they may be lighter, 

 or darker, or of nearly the same shade. 



When testing for colour blindness the following plan is recom- 

 mended. A pale green, a purple, and a red skein are chosen and 

 are termed test skeins. 



I. The green skein is first picked out. This skein should be the 

 palest shade of pure green, which is neither a yellow green nor a 

 blue green. The examination is continued until the examinee has 

 picked out all the other skeins of the same colour, or else placed 

 with them one or more skeins of what are termed the confusion 

 colours. These confusion colours which a colour-blind person will 

 thus pick out are of various tints, according to the amount of his 

 defect. Thus there may be greys, light reds, or light purples. The 

 fact that any confusion colour is picked out is sufficient to show that 

 he is colour blind. To determine the kind and degree of colour 

 blindness the next test skein is given. 



II. This is a purple skein, and should be midway in colour 

 between the lightest and darkest purple in the heap. The test is 

 continued until the examinee has picked out all the purples, or until 

 certain confusion colours have been selected. 



