LUMINOUS IMPRESSIONS ON THE EYE. 601 
which a portion of the image has already passed, and on which the light has had 
time to develop a distinct impression. In this manner, it may be 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 diferent Refrangibility act on the Eye with the same rapidity. 
In the observations made with a rapidly revolving disc, where each flash lasted 
only zs435 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 follows, that rays of light of 
different refrangibility 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 great 
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 moré 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 required for 
the transmission of impressions from the organs of sensation to the mind. 
VOL. XV. PART, V. TR 
