104 MICROSCOPIC OBJECTS. 



"The penetration of a microscope has been shown* 

 to be dependent on its angle of aperture, and that when- 

 ever this was less than a certain quantity, the lined struc- 

 ture of the scales cannot be rendered visible, however 

 perfect the instrument may be, and the defining power 

 is inversely as the quantity of spherical and chromatic 

 aberration. 



" In order to effect the union of these two properties it is 

 requisite not only that the aberration be destroyed for a 

 centrical pencil of rays, but that the whole of the pencils 

 within the field of view should be nearly perfect ; and it 

 is, I conceive, from this cause that no single lens, or sin- 

 gle object-glass, however perfectly achromatic or aplanatic, 

 wiU so far define an object that the lines and outline shall 

 be distinct at the same time — a property I have only seen 

 in sets of achromatic object-glasses. 



*' A proof or test object may be defined to be one 

 which requires a certain degree of excellence or perfec- 

 tion in a microscope or engiscope for the development 

 either of the whole or some particular part of its struc- 

 ture. Test-objects are separable into two great divisions ; 

 but as I intend only to treat on one of them, it is proper 

 here to point out their distinction. In the first division 



fifteenth focus) that shows the lines on the long brassica very distinct 

 and sharp, when its aperture is large, but will not define a moss 

 satisfactorily with this aperture ; but as stops behind the object 

 have the effect of reducing it, with them it shows the latter." 



* See introductory chapter to the third edition of the Microscopic 

 Illustrations. 



