CONSTRUCTION OF THE MICROSCOPE. 



21 



distance of the principal focus, the image being formed at 

 an equal distance on the other side of the lens, will be of 

 the same dimensions with the object, as in fig. 7 ; but if 



Fig. 7. 



the object be placed nearer to the lens, the image will be 

 farther from it, and of larger dimensions, as in fig. 8 ; and, 



Fig. 8. 



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. 



Aberration of Lenses. — Although the image of an object 

 produced by the convex lens, fig. 8, appears at first view 

 to be an exact reproduction of the object, it is found, 

 when submitted to rigorous examination, to be more or less 

 confused and indistinct : which is augmented when viewed 

 in a microscope. This indistinctness and confusion arises 

 from two causes, one depending on form, and the other on 

 the material of the lens. That which depends on the 

 form of the lens we shall now proceed to explain. 



