HAIR STRUCTURE OF THE MONOTREMATA 469 
the hair shaft. For the study of both. the cuticular scales and 
the medulla with the hair rotator, both reflected and transmitted 
light were employed. Were the device made in such a way that 
the hair was held more closely to the upper surface of the slide, 
no doubt both indirect (or dark-field illumination) and polarized 
light might also be utilized with advantage. For securing a pro- 
file view of the hair, with its serrated outline of the cuticular 
scales, the rotator was used with reflected light, the source of 
which was placed in front, slightly above and a little to the side 
of the microscope, and the slide beneath the hair covered with 
black unglazed paper. 
Another method of securing a profile view of the hair was to 
use indirect lighting or dark-field illumination, with the hair 
mounted dry. This method of illumination was best secured by 
the use of the dark-field illuminator, in which the central column 
of light is intercepted by an opaque disc in the condensing lens. 
For all dark-ground work it was found necessary to utilize a very 
brilliant light, especially with the higher powers. Because of 
the great degree of obliquity of the light required when using the 
1.8-mm. (or higher) objective, it was necessary to form an oil 
connection between the upper surface of the condensing lens and 
the lower surface of the slide. If this is not done, nearly all of 
the light passing through the condenser from the mirror will not 
be transmitted through the upper surface of the former, but will 
be reflected, leaving the object to be examined in darkness. The 
use of oblique light was found necessary in many cases to demon- 
strate to the examiner’s satisfaction the relations of the finer 
markings on the surface of many of the smallest hairs—markings 
that proved to be elusive under any of the methods of treatment 
heretofore described. 
In the study of the medulla the dark-field type of lighting was 
also found helpful, though here the grosser structure of this ele- 
ment of the hair shaft rendered such refined manipulation not so 
essential. 
In certain instances the polariscope was found to be an invalu- 
able aid, particularly in detecting the presence within the hair 
shaft of vestiges of medullary cells or groups of cells, and in indi- 
