260 SENSE OF SIGHT. 



to see distinctly in obscurity than others. This is owing to the retina 

 being more sensible; and, consequently, requiring a less degree of light 

 to impress it. On the other hand, a very powerful light injures the 

 retina, and deprives it, for a time, of its function; hence the unpleasant 

 impression produced by the introduction of lights into a room, where 

 the company have been previously sitting in comparative obscurity ; or 

 by looking at the sun. The effect upon the retina, thus induced, is 

 called dazzling. If the light that falls upon the eye is extremely 

 feeble, and we look long and intensely upon any minute object, the 

 retina is fatigued ; the sensibility of its central portion becomes ex- 

 hausted, or it is painfully agitated; and the objects appear and disappear, 

 according as it has recovered or lost its sensibility; a kind of remission 

 seeming to take place in the reception of the impressions. 



These affections are considered by Sir David Brewster 1 as the source 

 of many optical deceptions, which have been ascribed to a supernatural 

 origin. "In a dark night, where objects are feebly illuminated, their 

 disappearance and reappearance must seem very extraordinary to a 

 person whose fear or curiosity calls forth all his powers of observation. 

 This detect of the eye must have been often noticed by the sportsman, 

 in attempting to mark, upon the monotonous heaths, the particular 

 spots where moor-game had alighted. Availing himself of the slightest 

 difference of tint in the adjacent heaths, he endeavours to keep his eye 

 steadily upon it as he advances; but, whenever the contrast of illumi- 

 nation is feeble, he almost always loses sight of his mark, or if the 

 retina does take it up a second time, it is only to lose it again." 



In all the cases, in which the eye has been so long directed to a 

 minute object that the retina has become fatigued, on turning the axis 

 slightly away from the object, the light from it will fall upon a neigh- 

 bouring part of the retina, and the object be again perceived; and in 

 the mean time the part, previously in action, will have recovered from 

 its fatigue. By the fact of the retina becoming fatigued by regarding 

 an object for a long time we explain many interesting phenomena of 

 vision. If the eye be directed, for a time, to a white wafer laid upon 

 a black ground; and afterwards to a sheet of white paper, it will seem 

 to have a black spot upon it, of the same size as the wafer; the retina 

 having become fatigued by looking at the white wafer. On the other 

 hand, if the eye be turned to a black wafer, placed upon a sheet of 

 white paper; and afterwards to another part of the sheet, a portion of 

 the paper, of the size of the wafer, will seem strongly illuminated ; 

 the ordinary degree of light appearing intense, when compared with 

 the previous deficiency. It is on this, that the whole theory of acci- 

 dental colours, as they are called, rests. When the eye has been for 

 some time regarding a particular colour, the retina becomes insensible 

 to this colour; and if, afterwards, it be turned to a sheet of white 

 paper, the paper will not seem to be white, but will be of the colour 

 that arises from the union of all the rays of the solar spectrum, except 

 the one to which the retina has become insensible. Thus, if it be 

 directed for some time to a red wafer, the sheet of paper will seem to 



1 Op. citat., p. 250. 



