114 THE MICROSCOPIST. 



In examining test objects it will be well to remember that 

 there are generally some very easy ones, even among samples 

 of the most difficult kind. The darker the specimen, the more 

 easily is it made out ; and the more transparent the tissue, the^ 

 greater difficulty there is in developing its structure. Great 

 attention too should be paid to the proper illumination of the 

 object, or a superior instrument will be undervalued. 



The following list affords an account of those objects most 

 frequently used as tests of the defining power of the instru- 

 ment. 



BAT'S HAIR. This is a most beautiful structure, presenting 

 a series of scale-like projections arranged in the form of a 

 whorl around the central part or shaft. They are least nume- 

 rous at the base of the hair, and increase towards the apex. 



MOUSE HAIR differs materially from the other in size and 

 structure. Their internal structure is cellular, there being 

 three or more rows of cells in each hair, the color of the hair 

 depending on the pigment within the cells. Under the micro- 

 scope all hairs should have their light or transparent parts 

 clearly and distinctly separated from the darker portions, and 

 it is from the sharpness with which the parts are separated 

 that a correct opinion of the value of an instrument can be 

 obtained. 



In selecting hair of animals for examination, the lightest 

 colored should be preferred. Like the scales on insects, the 

 hair from different parts of the same individual varies consi- 

 derably in structure. 



HAIR OF THE DERMESTES. This very remarkable hair is 

 obtained from the larva of a small beetle, which preys on dried 

 animal substances, as bacon and hams. It is covered with 

 brownish hairs, the longest of which are selected. 



The shaft of this hair is covered with whorls of close-set 

 spines, and at the head is invested with a curious arrangement, 



