16 PROPERTIES OF LENSES. 



from colour, and their definition as perfect as those 

 at the centre. The illumination should be white, not 

 yellow, as is the case with inferior glasses, especially 

 those of foreign manufacture ; if of low power, that 

 is of one inch or more nominal focal length, opaque 

 objects should be clearly shown without the use of 

 any apparatus for condensing the light upon them. 



The rotundity of certain objects should come out 

 well, the more distant parts being in as good focus 

 as those nearer the objective. 



Transparent objects possessing a certain thickness, 

 should be clearly defined to a certain depth. 



This power of penetration is difficult to form a 

 correct opinion of, as we shall find that a glass of 

 given focus magnifies the depth of an object in the 

 proportion of the square of the magnification of the 

 diameter laterally. 



It is in inverse ratio to the magnifying power of 

 the objective. 



Therefore low powers are most suitable for the 

 display of those objects in which different points, 

 lying on different planes require to be in focus at the 

 same time. 



3. It has been found that the thin glass used for 

 covering microscopic specimens, produces by refrac- 

 tion of the rays passing through it a sufficient nega- 

 tive aberration (Fig. 11) to destroy the adjustment 

 of an objective perfect in its performance on un- 

 covered objects. 



This may be remedied, as has been shown by Mr. 

 A. Ross, by under-correcting the front and over- 



