82 COLOUR-SIGHT AND COLOUR-BLINDNESS 



and as the construction of the former and the uses of its 

 various parts and attachments are fairly well understood, 

 it shall be taken as the standard of comparison. The pro- 

 tective wood and leather of the camera have their homo- 

 logue (structure) and analogue (function) in the selerotic, the 

 tough, fibrous outer coat of the eye ; the optical part is 

 represented by the crystalline lens and other refractive media ; 

 the focussing gear by the ciliary muscles ; the iris-diaphragm by 

 the iris and pupil ; ihe time and instantaneous shutter by the 

 eyelids; the blackening (to prevent refections, etc.) by the 

 choroid, the pigmentary or middle coat of the eye ; and the 

 sensitive gelatine bromide of silver plate by the sensory retina, 

 the inner lining of the globe. 



The retina is exceedingly complex and although less than 

 j-^ inch in thickness is described as composed of ten layers, 

 the outer of which, Jacob's membrane, the bacillary or layer of 

 rods and cones, is the most important and is looked upon as 

 '' undoubtedly the true perceptive layer," for where rods and 

 cones are absent as at the entrance of the optic nerve, there 

 is the blind spot, and where the cones are thickest, as at the 

 yellow spot, there vision is most acute. 



The resemblance to photography goes still further, and 

 assumes a photo-chemical or photo-chemico- vital aspect ; for 

 just as the sensitive plate undergoes photo-chemical changes on 

 exposure to light, so in the eye is a substance which is 

 similarly affected. This is a colouring matter or pigment 

 which coats the tips of the rods — the visual purple or rhodopsin, 

 and is quickly bleached by light to be restored by rest and 

 darkness. And the photographic comparison is completed — for 

 as the exposed plate can be developed to yield a picture and then 

 fixed, so permanent optograms can be obtained from the retina. 

 " That images of objects can be formed on the retina owing to 

 the bleaching of the visual purple has been proved by experiment. 

 The purple is first changed to a yellow colour and then passes 

 into white. These optograms, as they are called, can be fixed 

 in an excised eye if the retina be detached and then treated 

 with a weak solution of alum." 



The particular function of this purple has not been 

 determined ; it is confined to the outer segments of the rods ; 

 the cones are devoid of it and as there are but cones in the 

 area of most acute vision, it is difficult to understand how 

 sight can be dependent upon this retinal purple. 



