LUMINOUS IMPRESSIONS ON THE EYE. 601 



which a portion of the image has already passed, and on which the hght has had 

 time to develop a distinct impression. In this manner, it may he conceived, that 

 the breadth of the image will be diminished on the side towards which it moves, 

 while it will be increased on the other side by the persistence of the impression 

 of light on the eye ; and, consequently, the image of the star will appear behind 

 its true position. It is obvious, however, that the retardation of the advancing 

 edge of the image cannot exceed the breadth of the extremely minute disc with 

 which a star appears in a good telescope, otherwise it would amount to a total 

 extinction of the light ; and, on the other hand, the image cannot be prolonged 

 by the persistence of its impression on the retina, by a greater quantity than its 

 advancing edge is retarded, without becoming perceptibly elongated. Any dif- 

 ference in the amount of retardation due to such causes, in different eyes, must 

 therefore be confined within extremely narrow limits, and seems quite inadequate 

 to account for the personal equation which, in some instances, amounts to a large 

 fraction of a second.* 



2. Rays of Light of different RefrangiMlity act on the Eye with the same rapidity. 



In the observations made with a rapidly revolving disc, where each flash lasted 

 only 18532 of a second, not the slightest alteration in the colour of the luminous 

 object was perceptible. The blue part of a gas flame, indeed, became invisible ; but 

 this was evidently due to the great reduction of the intensity of the light render- 

 ing the blue rays incapable of producing a sensible impression on the eye, already 

 affected by the more luminous rays. From this it foUows, that rays of light of 

 different refi-angibility act on the eye with equal rapidity. For if we suppose some 

 of the rays which constitute white light to act on the eye more rapidly than 

 others, the effect of shortening the luminous impressions would be quite analogous 

 to that produced by the interposition of some medium, such as red glass, which 

 absorbs the rays unequally ; and the eye would be affected with the complemen- 

 tary colour of the deficient rays. 



That there is no sensible difference in the rapidity of the action of lights of 

 various colours on the retina, appears also from the fact, that when the eye is sud- 

 denly directed to a luminous object, the first impression of its colour remains 

 afterwards unaltered. This could not be always the case if there was any gi-eat 

 inequality in the rapidity with which the different rays produce their effect on 

 the eye. If, for example, we suppose the blue rays to act more rapidly than 

 the yellow rays, green objects would, at first sight, appear to have mor6 of a bluish 

 tinge than after the eye had continued to regard them for a short time. 



* As my object here is simply to discuss the possibility of explaining the personal equation by 

 the gradual action of light on the retina, I have intentionally refrained from entering upon any ex- 

 planation of that phenomenon which may be derived from the supposition that time is requii-ed for 

 the transmission of impressions from the organs of sensation to the mind. 



VOL. XVI. PAET v. 7 E 



