Elements of animal physiology, chiefly human . or colourwill be experienced. 368. Colour, like sound, is thus a sejisation consequenton brain-change produced by the transmission of a movement or change in the substance of the nervefibrils to the brain, the movement being originated by 5^HE EYE. 1G5 an external vibrating agent. In the case of sound, theexternal agent is the aio%- in the case of light or colour,it is the luniiniferous ether. 369. Colour Blindness, or Daltonism, consists inthe inability of certain eyes to distinguish particularcolours. Singularly the most common defec
Elements of animal physiology, chiefly human . or colourwill be experienced. 368. Colour, like sound, is thus a sejisation consequenton brain-change produced by the transmission of a movement or change in the substance of the nervefibrils to the brain, the movement being originated by 5^HE EYE. 1G5 an external vibrating agent. In the case of sound, theexternal agent is the aio%- in the case of light or colour,it is the luniiniferous ether. 369. Colour Blindness, or Daltonism, consists inthe inability of certain eyes to distinguish particularcolours. Singularly the most common defect of thisis the inability to distinguish red from black, green,&c. It is not yet known whether this weakness arises from a defectin the brain, the retina, or the humours of the eye. The employ-ment of colour-blind personsas railway guards might lead to mostserious accidents. 370. The Eye is essentially an optical instrument,constructed for receiving, bending (refracting), and thro\y- Conjuiiotiva. .. .^ .Crystalline Lens. /Sclerotic *ClioroiJ Coat. Ciliary Processes. Fig. 77. Showing the formation of inverted optical images on theEetina at the back of tke Eye. ing the rays of light on to a screen (the retina) at its back:,so that it shall receive a very minute and inverted, butclear and definite, 2yicture or image of the surroundingobjects. In fact, in no case do we see the externalobjects themselves, but pictures of them formed by ilielight sent from them, and focussed on the back of the•eye (the retina), as just described. The eye is, infact, a sort of water camera obscura: it is moved bysix muscles attached to its external coat (the sclerotic).The eyes are lodged, for protection, in packings of fatin the orbits of the cranium. 166 ANIMAL PHYSIOLOGY. 371. structure of the Eye.—The eye i« a nccaily roundball, about 1 inch in diameter, which encloses threelenses or humours and tv^o muscles, and which consistof three coats or layers. It also contains nerves and blood
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