Vision and Optical Glasses. 195 



What is thus established in theory may be de- 

 monstrated by experiment, and that experiment 

 is easy to make. Cover one side of a glass globe 

 or of a thick lens with a piece of brown paper, 

 making a row of pin-holes across the diameter of 

 the lens very accurately at equal distances. Let 

 the light which passes through the lens fall upon 

 a sheet of white paper, and you will find that 

 when the paper is held near the lens the spots of 

 light will be nearly at equal distances ; but if the 

 paper is further removed, the intervals between 

 the exterior spots become less than the intervals 

 between the interior, and soon unite. 



But there is a still further aberration, which is 

 productive of even a greater inconvenience than 

 this which I have now specified. When I come 

 to treat of the prism and the prismatic colours, 

 you will find that each particle of light is suscep- 

 tible of a different degree of refrangibility, and 

 consequently that every lens (especially high 

 magnifiers) acts in some degree as a prism in 

 separating the different coloured rays Hence, if 

 we suppose PP (fig. 75) to be a double convex 

 lens, and oo an object at some distance from it, 

 if the object oo were red, the rays proceeding from 

 it would form a red image Rr ; if it were violet, 

 an image of that colour would be formed at \v 

 nearer the lens ; and if the object were white, or 

 any other combination of different coloured rays, 

 these rays would have their respective foci at dif- 

 ferent distances from the lens, and form in fact 



