336 Mr. Pritchard's Account of Test Objects for Microscopes, 



bited with the greatest perspicuity. In this investigation, it 

 was found that there were two distinct properties in a micro- 

 scope, and that the instrument might possess a very consider- 

 able approximation to perfection in the one, and tall short in 

 the other, or vice versa, or might be perfect in both. The 

 lines on the dust or feathers from the wings of the lepidoptera, 

 and those on the scales from the body and limbs of the thysa- 

 nuraeous insects, offered the means of determining their good- 

 ness in one particular, viz. their penetration, and the structure 

 of the hair of animals, certain mosses, &c. served to ascertain 

 their defining power. 



The analogy between telescopes and microscopes is so great, 

 that I cannot be said to digress from my subject by stating 

 that the aforesaid observations apply also to the former of these 

 instruments, which seldom combines the two qualities of pene- 

 tration and definition to any great extent. Thus, a telescope 

 with a large aperture will frequently resolve clusters of stars, 

 and exhibit nebulae, while it will fail in defining the disc of a 

 planet, or the moon, with precision ; and, on the other hand, 

 one of moderate aperture accurately figured will define the 

 latter, but be wholly inert on the nebulae and clusters. So a 

 microscope with large aperture and high power will show the 

 u active molecules" and lined objects, while it will not define 

 a leaf of moss, or a mouse hair ; and another with a smaller 

 aperture will define the latter, but prove ineffective on the 

 former. This is very manifest in single lenses which require 

 different apertures for different objects*. 



The penetration of a microscope has been shown to be de- 

 pendent on its angle of aperture, and that whenever this was 

 less than a certain quantity, the lined structure 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. 



A proof, or test-object, may be defined to be one which re- 

 quires a certain degree of excellence or perfection in a micro- 

 scope or engiscope for the development either of the whole, or 

 some particular part of its structure. 



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 I place those which 

 operate out of focus, and tell us what the defects of an instru- 

 ment are. The second, those which, if exhibited by a micro- 



* I have a very beautiful sapphire lens (plano-convex of one 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. 



