FUNCTION OF ROBS AND CONES 429 



shock sent through the eyeball may give rise to the 

 appearance of a flash of light : and pressure on any 

 part of the retina produces a luminous nnage, Av^nch 

 Lts as long as the 'pressure, and is called a phos- 

 phene. If the point of the finger be pressed upon the 

 outer side of the ball of the eye, the eyes being shut, a 

 luminous image-which, in most cases, is dark m the 

 centre, with a bright ring at the circumference (or as 

 Newton described it, like the "eye" in a peacocks taU- 

 feather)-is seen ; and this image lasts as long as the 

 pressure is continued. i • j iu 



The sensation of light is, as already explained, the 

 work of those parts of the brain which, as the visual 

 sensorium, respond to the impulses reaching them 

 through the optic nerve. The retina is the means of 

 supplying the impulses to the sensorium and may be made 

 to do so by light ordinarily, but also by other kinds of 

 stimulation. But the visual sensorium itself may at 

 times be aflFected by influences other than those which 

 reach it from the retina. In this case also (subjective) 

 luminous sensations of the most vivid and startling kind 

 may be experienced, which give rise to delusive judgments 

 of the most erroneous kind (see p. 442). 



12 The Functions of the Rods and Cones.- i he 

 last paragraph raises a distinction between the - hbres of 

 the optic nerve "and the " retina " which may not have 

 been anticipated, but which is of much importance. 



We have seen that the fibres of the optic nerve raniify 

 in the inner fourth of the thickness of the retina, while 

 the layer of rods and cones forms its outer fourth, ihe 

 liaht therefore, must fall first upon the fibres of the optic 

 nerve, and, only after traversing them, can it reach the 

 rods and cones. Consequently, if the fibrillar of the optic 

 nerve themselves are capable of being aftected by hght, 

 the rods and cones can only be some sort of supple- 

 mentary optical apparatus. But, in fact, it is the rods and 

 con«s which are aflected by light, while the fibres of the 



