WORK OVER DRY MOUNTS, ETC. 305 



much as was the former case. They indeed appear in 

 good relief, but they do not seem to float clear from 

 the body of the scale; these appearances are to be 

 watched under the microscope, and the obliquity of the 

 illumination diminished to the least angle without 

 allowing the spines to rise. When the adjusting collar 

 and the illumination are just in the correct position, 

 the small end of the " wedges" or " exclamation points 

 are to be sharply defined. These scales may be exam- 

 ined either in a vertical or horizontal position ; as a 

 general thing the vertical will be the preferable posi- 

 tion. Most mounts of podura will contain scores of 

 scales, while for the most part there will be less than a 

 half-dozen good ones. The student should look care- 

 fully to this, and make the best selection possible. Ke- 

 ferring to the use of the vertical illuminator, Mr. 

 Morehouse called my attention to a curious fact con- 

 nected with the study of these scales, namely, that the 

 smallest and most uninviting of these, as shown by 

 transmitted light, seemed under the vertical illumina- 

 tor quite as strong, vigorous, and satisfactory as any. 

 We found Mr. Morehouse's observation true as respect 

 to many other insect scales we had occasion to examine. 



The following, mounted dry, will form useful objects 

 and may be advantageously studied conjointly with the 

 podura : 



Scales of Lepisma saccharina Degeeria Macrotoma 

 major Petrobius maritimus Pontia brassica Morpho- 

 menelaus Tinea vestimenti Gnat Hipparchia Janira 

 Wing of Gnat any of the scales from the lepidop- 



20 Microscopy. 



