254 PEACTICAL PHYSIOLOGY 



If a red blind person be examined as to his sensations along the 

 range of the spectrum, he sees nothing at the extreme red end of the 

 spectrum at all. A glimmer of what he calls dark green is seen in the 

 position of the red lithium line, and this green gradually becomes more 

 conspicuous to him through the yellow to the proper green. Passing 

 to the blue green he says the colour is grey, being similar to his idea of 

 white admixed with a certain amount of black. Passing further to the 

 blue end he recognises the blue and speaks of the violet as dark blue. 

 Similarly, a green blind | erson will recognise a grey in the middle of the 

 spectrum, but rather more in the green than the locality thus named by 

 the red blind. 



Colour blindness can be conveniently tested by the use of a series of 

 coloured wools of great variety of hue and tint. Such a set of wools are 

 spoken of as Holmgren's wools. The method, however, is not a safe test. 



EXPERIMENT. Spread out the wools on white blotting paper in a 

 good light. Avoid mentioning the names of the colours of any of these 

 wools, but pick < ut a whitish green, and request the subject to collect all 

 those wools which approximate in hue or tint to the colour presented. 



If any errors are made, proceed to test whether he is red blind, green 

 blind, or violet blind. Give him a skein of a magenta hue. If he is 

 red blind he will pick out blue and violet ; if green blind he will con- 

 fuse green and grey. 



The matching of colours may be also carried out by rotating the 

 various cards of the colour-mixer, and thus matches of any colour 

 under examination can be obtained. The same result can be obtained 

 by projecting various portions of the spectrum as mentioned in colour 

 mixing. 



CHAPTER LXX. 

 BINOCULAR VISION. 



THE images formed on the two retinae of an external object amongst 

 its surroundings will not be identical. The lack of identity enables an 

 observer to form a judgment" as to its position in space. Such a judg- 

 ment is more easily formed when the object is comparatively near than 

 when far off, as in this latter case 'the images are approximately similar. 

 Though the images for objects at a certain distance are not identical, it 

 is necessary that they should be thrown on certain corresponding parts 

 of the retina in order that a single sensation should result. 



In order that a single image then should result, it is necessary that 



