PROPERTIES OF LENSES. 15 



front, corrects all the others. In this combination 

 only five lenses are used, and the errors arising from 

 the sixteen surfaces of glass in the older forms re- 

 duced to ten. 



A good objective should possess the Standard 

 screw, adopted by the various societies, and now 

 fixed by the best makers to their lenses. 



Its definition should be clear, the field flat, objects 

 at the edge of the field should be as free 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 pro- 

 portion 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. 



