ON VISION. 357 



being long detained in the dark winding passages, which lead to the place 

 <5f exhibition. 



The impressions of light on the retina appear to be always in a certain 

 degree permanent, and the more so as the light is stronger ; but it is uncer- 

 tain whether the retina possesses this property merely as a solar phosphorus, 

 or in consequence of its peculiar organization. The duration of the im- 

 pression is generally from one hundredth of a second to half a second, or 

 more ; hence a luminous object revolving in a circle makes a lucid ring ; 

 and a shooting star leaves a train of light behind it, which is not always 

 real. If the object is painfully bright, it generally produces a permanent 

 spot, which continues to pass through various changes of colour for some 

 time, wiiVut much regularity, and gradually vanishes : this may, how- 

 ever, be considered as a morbid effect. 



When the eye has been fixed on a small object of a bright colour, 

 and is then turned away to a white surface, a faint spot, resembling in 

 form and magnitude the object first viewed, appears on the surface, of a 

 colour opposite to the first, that is, of such a colour as would be produced 

 by withdrawing it from white light ; thus a red object produces a bluish 

 green spot ; and a bluish green object a red spot. The reason of this 

 appearance is probably that the portion of the retina, or of the sensorium, 

 that is affected, has lost a part of its sensibility to the light of that colour, 

 with which it has been impressed, and is more strongly affected by the 

 other constituent parts of the white light. A similar effect is also often 

 produced, when a white, or grey object is viewed on a coloured ground, 

 even without altering the position of the eye : the whole retina being 

 affected by sympathy nearly in the same manner as a part of it was 

 affected in the former case. These appearances are most conveniently 

 exhibited by means of the shadows of objects placed in coloured light : the 

 shadow appearing of a colour opposite to that of the stronger light, even 

 when it is in reality illuminated by a fainter light of the same colour. It 

 seems that the eye cannot perfectly distinguish the intensity of a colour, 

 either when the light is extremely faint, as that of many of the fixed stars, 

 which Dr. Herschel has found to be strongly coloured, or when the light ' 

 is excessively vivid ; and that when a considerable part of the field of vision 

 is occupied by coloured light, it appears to the eye either white, or less 

 coloured than it is in reality : so that when a room is illuminated either 

 by the yellow light of a candle, or by the red light of a fire, a sheet of 

 writing paper still appears to retain its whiteness ; and if from the light of 

 the candle we take away some of the abundant yellow light, and leave or 

 substitute a portion actually white, the effect is nearly the same as if we 

 took away the yellow light from white, and substituted the indigo which 

 would be left : and we observe accordingly, that in comparison with the 

 light of a candle, the common daylight appears of a purplish hue. (Plate 

 XXX. Fig. 439.. .441.) 



LECT. XXXVIII.-ADDITIONAL AUTHORITIES. 



Vision. Fabricius ab Aquapendente, fol. Yen. 1600. Scheineri, Oculus 4to 

 Rom. 1652. Cherubin, Vision parfaite, 1678. Briggs, Ph. Tr. 1683, p. 17l! 



