TfiE SENSES 561 



not only with flashes of light, but to what has been termed 

 ' flashes of darkness/ If an excised eye be suitably connected to 

 a galvanometer, the eye being kept in the dark, an electrical 

 change occurs when light is transmitted through the pupil, and 

 there is a similar electrical change when the light is withdrawn. 

 The sensitiveness of the eye to these reactions increases by its 

 being kept in the dark. Not all ether-waves are capable of being 

 perceived. The eye, excellent as it is, is not free from optical 

 defects, as we have seen at p. 545. Similarly, the retina is not 

 sensitive for those red or violet rays known as ultra, though the 

 photographic plate is capable of recording them. 



A photo-chemical theory of vision, based on the ready de- 

 composition of visual purple, has been proposed. The visual 

 purple is limited to the external segments of the rods, the cones 

 contain none ; under the influence of light the pigment is bleached, 

 and in this way a retinal photograph, or optogram, may be ob- 

 tained in the eye of the frog or rabbit which has been suitably 

 prepared by being kept in the dark. If the pigment be extracted 

 and brought into solution, as it may through the action on it 

 of bile-salts, the coloured solution thus obtained is bleached on 

 exposure to light. There is good reason for thinking that some 

 such change occurs in the living eye. It is known that the 

 restoration of colour to the rods is effected by contact with the 

 pigment of the choroid. Nevertheless, this pigment is not neces- 

 sary to vision, as in the eyes of some animals it is absent, notably 

 in the hen and pigeon ; further, it is absent from the area of acute 

 vision in man — viz., the fovea. 



An eye adapted to light and an eye adapted to darkness is a 

 sensation with which all are familiar on passing from a light to 

 a dark room ; the vision improves by waiting in the dark, and it 

 can be shown in man that the improvement does not occur over 

 the cone area — viz., the fovea — but over the peripheral field, 

 where the rods predominate. The explanation of the vision 

 of night-seeing birds and animals probably lies in this function 

 of the purple-coated rods ; while acute daylight vision is probably 

 the essential function of the non-pigmented cones. Neither the 

 hen nor the pigeon, whose habits are diurnal, can be regarded as 

 possessed of eyes defective in acuteness of vision, yet neither 

 possess visual purple. On the other hand, neither possess vision 

 suitable for night-time. In the bat visual purple has been found ; 

 in the owl it exists in abundance. It seems justifiable to believe 

 that the difference between eyes suited for daylight and darkness 

 largely hinges on the question of visual purple, by which the 

 irritability of the rods is increased in dim light, and vision is 

 facilitated in illuminations of low intensity. Too little is known 

 of the tapetum lucidum to enable its function in connection with 



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