Mr. Pritchard's Account of Test Objects for Microscopes. 343 



are worthy of investigation*, I have sketched a few of the 

 most interesting varieties. They are all magnified in the same 

 proportion as the mouse and bat's hairs, which accompany 

 them in Plate III. 



Fig. 20. is a hair from the larva of the common der- 

 mestes. 



Fig. 21. is a white hair from a young cat. 



Fig. 22. is the hair of a Siberian fox; and 



Fig. 23. the hair of a common caterpillarf. 



4. The Lyccena Argus. — Among the scales on the underside 

 of the wing of this elegant blue butterfly, are some whose con- 

 formation is remarkably singular; their form is represented in 

 fig. 24 ; their general appearance is not unlike a child's bat- 

 tledore, with its surface covered with spots. I have not been 

 able satisfactorily to demonstrate its structure; but it appears 

 io consist of two delicate tissues, having regular rows of coni- 

 cal spines on the upper one. As a test-object, these spots 

 should be clearly and distinctly separated. When the light 

 is thrown obliquely, they are blended together, appearing like 

 a stripe of unequal breadth ; similar to many of the other tests, 

 it is the manner in which they are seen rather than the mere 

 exhibition of them that should be observed. This object I 

 employ for the same purpose as the leaf of an unknown spe- 

 cies of moss belonging to the genus Hypnum, which, as it is 

 difficult to procure, renders this substitute an acquisition to 

 the microscopist. 



Before I conclude this chapter, it may not be amiss to no- 

 tice another class of objects, which by the vulgar are consi- 

 dered as positive proofs of the efficiency of an instrument; I 

 allude to the animalcules. Nor does this opinion seem con- 

 fined to those unacquainted with this subject; but we find it 

 stated by Adams, in his quarto work on the Microscope, p. 430, 

 that the Monas Termo (one of the most minute of all the ani- 

 malcules very abundant in vegetable infusions), "eludes the 

 power of the compound microscope, and is but imperfectly 

 seen by the single." Now, all that is requisite for seeing this 

 object, or any other of the same kind, is to cut off by stops, 

 or otherwise, all superfluous light, so as to reduce the quantity 

 and intensity^ of the illumination ; for, when too much light 



* The serratures on the surface of the human hair, especially those from 

 an infant, afford excellent tests, and are very beautiful objects. 



f The form of these hairs varies in every species: in some they resemble 

 the feather of the peacock's tail in miniature; others are furnished with 

 tufts of fine hair, and beset with spines. 



X The reader should observe that quantity and intensity are distinct from 

 each other: thus, when we employ a small wax taper close to an object, it 



