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 l-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 l-4th to 1-1 Oth 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 



