290 Dr Brewster on some Affections of the Ret hid. 



If the slip of white paper, viewed indirectly with both eyes', 

 is placed so near as to be seen double, the rays which proceed 

 from it no longer fall on corresponding points of the retina. 

 In this case, the two images do not vanish simultaneously ; but 

 when the one begins to disappear, the other begins soon after 

 it, so that they sometimes appear to be extinguished at the 

 same time. 



In order to ascertain whether or not the accidental colour of 

 an object seen indirectly would remain after the object itself 

 had disappeared, I placed a rectangular piece of a red wafer 

 upon a white ground, and having looked steadily at an object 

 in its vicinity, the wafer disappeared, and though the acciden- 

 tal colour showed itself just before the wafer had vanished, 

 yet no trace of colour was visible afterwards. 



The insensibility of the retina to indirect impressions has a 

 singular counterpart in its insensibility to the direct impres- 

 sions of attenuated light. When the eye is steadily directed 

 to objects illuminated by a feeble gleam of light, it is thrown 

 into a condition nearly as painful as that which arises from an 

 excess of splendour. A sort of remission takes place in the 

 conveyance of the impressions along the nervous membrane ; 

 the object actually disappears, and the eye is agitated by the 

 recurrence of excitements which are too feeble for the perform- 

 ance of its functions. If the eye had, under such a twilight, 

 been making unavailing efforts to read, or to examine a minute 

 object, the pain which it suffers would admit of an easy ex- 

 planation ; but, in the present case, it is the passive recipient 

 of attenuated light, and the uneasiness which it experiences 

 can arise only from the recurring failures in the retina to 

 transmit its impressions to the optic nerve. 



The preceding facts respecting the affections of the retina, 

 while they throw considerable light on the functions of that 

 membrane, may serve to explain some of those phenomena of 

 the evanescence and reappearance of objects, and of the 

 change of shape of inanimate objects, which have been ascrib- 

 ed by the vulgar to supernatural causes, and by philoso- 

 phers to the activity of the imagination. If in a dark night, 

 for example, we unexpectedly obtain a glimpse of any object, 



