520 



THE AMERICAN NATURALIST [Vol. LIV 



with the 1.8 mm. objective and 4x eyepiece was found ex- 

 cellent for a large number of hairs. It must be borne in 

 mind that, in using this combination of immersion ob- 

 jective with the dark held illuminator, an oil connection 

 must also be formed between the upper surface of the 

 condenser and the lower surface of the slide. 8 Exigency 

 of space forbids the descriptions of the various types of 

 lighting which have been found most satisfactory with 

 the various species of hairs. These must be empirically 

 determined by each investigator. The degree of success 

 obtained with the microscope usually depends as much 

 upon the preparation of the instrument and its lighting, 

 as upon the preparation of what is to go under it for ex- 

 amination. 



For examination of the medulla all that has been said 

 regarding lighting, etc., applies. However, the various 

 treatments given the hair and used to render visible the 

 cuticular scales, obscure the medulla. The simplest and 

 most generally useful method of rendering the medulla 

 clear, consists in lvducinu' the visibility of the cuticular 

 scales to as near zero as possible by mounting the hair, 

 beneath the cover glass, in some light microscopical oil, 

 such as oil of bergamot, of cedar, or origanum, of amber, 

 of cloves, etc., after having washed it, as before, in the 

 ether-alcohol solution. Such a treatment renders the 

 hair, in effect, a glassy cylinder, within which the medulla 

 can be clearly Been, provided the cortex is not thickly be- 

 sprinkled with pigment granules, or rendered dark in 

 color by diffuse pigment. Fortunately most of the fur 

 hairs are lightly pigmented. Many of the protective 

 hairs, however, are so heavily colored that the medulla 

 is partially, or almost wholly, obscured. 



Some of the finer hairs can be examined with advan- 

 tage in a mount of clear water, or xylol. The best treat- 



