438 DURATION OF LUMINOUS IMPRESSIONS. 
a child approaching us appears to have the size of a man, and 
a man seems like a giant ; for the indistinctness of the outline 
excites in the mind the idea of distance; and the same pic- 
ture, if supposed to be that of a more remote object, will give 
rise to the idea of greater size. The want of innate power in 
Man to form a true conception of either size or distance, is 
well shown by the effect produced on the mind unprepared 
for such illusions, by a skilfully-painted picture, the view of 
which is so contrived that its distance from the eye cannot be 
estimated in the ordinary manner; the objects it represents 
being invested by the mind with their real sizes and respective 
distances, as if their real images were formed upon the 
retina. This illusion has been extremely complete in some 
of those who have seen the panoramic view of London in the 
Colosseum, 
567. When a number of luminous impressions are made 
upon the retina at short intervals, they become blended into 
one,—the intervals being undistinguishable. Thus, when we 
rapidly move the end of a lighted stick in a straight line or 
circle, the impression produced is that of a band or ring of 
light ; for the impression made by the light, as it passes each 
point, remains for some time subsequently. If the stick be 
whirled round with sufficient rapidity for it to reach any 
point a second time, before the impression made by its pre- 
vious passage has departed, an unbroken circle of light is 
produced. By experiments made in this manner, we may 
determine the longest interval during which visual im- 
pressions remain on the sensorium ; for if we find that a hot 
coal, whirling round at the rate of six times in a second, 
produces a continued circle of light, but that the circle is 
broken when it turns round only five times in a second, we 
know that the length of the impression is 1-6th of a second. 
By experiments of this kind, it has been found that the 
duration varies in different individuals, and in the same 
individual at different times, from 1-4th to 1-10th of a 
second. On this principle several very ingenious toys have 
been constructed, in which two or more images are com- 
bined, by the rapid revolution of a wheel on which they are 
painted. 
568. Some persons, whose sight is perfectly good for forms, 
distances, &c., are unable to discriminate colours. This is} 
