THE PHYSICAL BASIS 7 



light is always more or less impure. The nearest approach is given by 

 the following list : 



Red — vermilion (not scarlet vermilion) mixed with a small quantity of permanent 

 violet. 



Orange — orange cadmium. 



Yellow — chrome yellow. 



Green — Prussian blue mixed with aurelin. 



Blue-green — viridian mixed with a small amount of cobalt blue. 



Blue — ultramarine. 



Violet — permanent violet mixed with a small amount of blue, (Abney.) 



CHAPTER II 



THE ANATOMICAL BASIS 



I do not propose to discuss fully the anatomy and physiology of the 

 eye and visual paths, but it is necessary to draw attention to certain 

 features of special importance in colour vision. This course will doubt- 

 less emphasise the great complexity of the subject, which is too often 

 wilfully ignored. 



The eye resembles a photographic camera, in which the cornea and 

 crystalline lens represent the lens-system, the iris the diaphragm, and 

 the retina the sensitive plate. The size of the pupillary aperture is not 

 under voluntary control, but varies with the intensity of light entering 

 the eye and other causes. This fact has to be taken into consideration 

 in some experiments. (The reader is recommended to read the earlier 

 chapters in the author's Manual of Diseases of the Eye, 2nd edition, 

 J. and A. Churchill, London, 1912.) The optical system of the normal 

 eye at rest is focussed for distant objects, i.e. parallel rays are brought 

 to a focus upon the retina. Focussing for near objects is brought about 

 by automatically altering the " strength " of the crystalline lens (accom- 

 modation), not by altering the length of the eye as in the photographic 

 camera. Some eyes are naturally too short or their optical system at 

 rest is too weak, so that accommodation is required even for distant 

 objects (hypermetropia). Others are too long or their optical system 

 at rest is too strong so that it is impossible to focus distant objects with- 

 out the aid of concave spectacles and near objects may be seen clearly 

 without the aid of accommodation (myopia). Many eyes show slight 

 differences in the radius of curvature of the cornea in meridians at right 

 angles to each other and this is often associated with slight tilting of the 

 lens (regular astigmatism). The lens always shows slight irregularities 



