CHAPTER VII. 



TEST OBJECTS. 



THE discovery of this class of objects by Dr. Goring, a full 

 account of which may be found in Mr. Pritchard's works on 

 the Microscope, was the chief cause of the modern improve- 

 ments in the achromatic compound microscope. 



Mr. Pritchard, following Dr. Goring, divides test objects 

 into two classes, viz., tests of the penetrating power, and tests 

 of the denning power of the instrument ; the first showing its 

 destitution of spherical and chromatic aberration, and mechani- 

 cal imperfection; and the other class showing its angle of 

 aperture. 



This distinction is not now necessary, as few persons, save 

 those engaged in the manufacture of object-glasses, attend to 

 the former, the improvement in achromatic object-glasses hav- 

 ing been so extensive that a good instrument, in this respect, 

 is readily procurable. Still, it may be well to give an outline 

 of the means by which the presence or absence of achromati- 

 city may be known. 



Chromatic aberration is rendered sensible by almost any 

 transparent object, when the light falls upon it obliquely; but 

 more especially by such as are not transparent, but only illu- 

 minated by intercepted light, of which a very good example 

 may be seen in a piece of fine wire sieve, treated like a dia- 

 phanous object, also in a thin plate of metal perforated by very 

 small holes. The various colors are seen according to the 



