CONSTRUCTION OF THE MICROSCOPE. 17 



of the lens, will be of the same dimensions with the object, as in fig. 7 ; 



fig. 7. 



but if the object be placed nearer to the lens, the image will be farther 

 from it and of larger dimensions, as in fig;. 8 ; and on the other hand, 



if the object be farther from the lens, the image will be nearer to it 

 and smaller than itself. But it is to be observed, that the larger the 

 image is in proportion to the object, the less bright it will be, because 

 the same amount of light has to be spread over a greater surface; 

 whilst a smaller image will be much more brilliant. 



Spherical Aberration of Lenses. 



We have many imperfections to contend with in our optical arrange- 

 ments and combinations ; one of which results from the spherical aber- 

 ration of the rays passing through lenses whose curvatures are equal 

 over their whole surfaces. If the course of the rays be observed, it 

 will be seen that they do not all meet exactly in the foci already stated, 

 but that the focus of the ray& which have passed through the circum- 

 ferential portion of the lens is much closer to it than that of the rays 

 which are nearer the line of its axis. This is shown in fig. 9 : a b, the 

 rays, are seen falling on the circumference and coming to a focus, /; 

 a b', a" b" are rays falling nearer the centre, and coming to a more dis- 

 tant focus,/"; so that if a screen be held at cd, the rays which have 

 passed through the central portion of the lens will be stopped at /be- 

 fore they come to a focus, or they will come to it in a state of diverg- 



c 



