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



pear to be essential for the development of their structure, the 

 degree of obliquity varying, however, with different specimens 

 of scales. The extremes of this variation are the Podura 

 plumbea and Pieris Brassica, the delicacy of the former re- 

 quiring almost central light, while the latter requires it very 

 oblique. From this cause artificial illumination is to be pre- 

 ferred to day-light for this class of objects, as the light of a 

 lamp or candle gives the rays diverging without any apparatus 

 whatever. The same effect, however, may be produced in 

 day-light, either with Dr. Wollaston's or Dr. Goring's illu- 

 minator, where the rays, after meeting at the focus of their 

 illuminating lens, are permitted to diverge, and, by placing 

 the object out of the centre, oblique vision is obtained. In the 

 investigation of the class of objects now to be described, direct 

 parallel rays are preferable, and, indeed, in most cases are es- 

 sential ; and on this account they are scarcely ever well defined 

 by candle- or lamp-light. In these, therefore, clear day-light, 

 directed through the axis of the instrument, should be em- 

 ployed. 



1. The hairs of the common mouse (Mus domesticus) differ 

 both in size and form ; the principal varieties, with their rela- 

 tive diameters, are shown in Plate III. fig. 14, 15, and 16. 

 These are drawn, as seen by transmitted light, and as proof- 

 objects should have their transparent parts clearly and dis- 

 tinctly separated from the darker portions. This remark 

 holds good for the whole tribe of hairs and mosses, and it is 

 from the sharpness with which the parts are separated that a 

 correct opinion of the goodness of an instrument can be ob- 

 tained. When these hairs are seen by reflected light, that is, 

 as opake objects, their appearance is altered, the dark solid 

 parts reflecting more light than the transparent portion ; hence 

 they are lighter than the latter. A peculiar and interesting 

 variety of a large hair viewed in this way is shown at fig. 17; 

 it is engraved from a drawing made by Dr. Goring. * * * 



3. The hair from the wing of the bat {Vespertilio murinus), 

 — Although this creature is supposed to bear some affinity to 

 that of the mouse, the structure of the hair of these two ani- 

 mals is entirely different : there are, however, great varieties, 

 the principal of which are shown at fig. 18. and 19. The 

 hair in the latter figure is spiral; the former like a succession 

 of cones, the apex of one being inserted into the base of the 

 following. 



Many other kinds of hair might be enumerated for the pur- 

 pose to which I have applied the above; but I deem these 

 amply sufficient to illustrate this part of the subject. As, how- 

 ever, the diversities in the structure of different kinds of liaif 1 



