Oblique Illumination. i r 



We can thus secure the accuracy so much desired. 



As soon as our standard is obtained we should take steps to 

 have it deposited with the bureau of weights and measures at 

 Washington, for preservation and reference. 



In order to avoid the annoyance of converting fractions of 

 an inch or line into the new scale by calculations, the society 

 should publish a table of such reductions, compiled with the 

 utmost care, together with such rules as may seem useful in 

 practical work. 



I believe it is not necessary for me to enlarge upon the real 

 benefits to be thus secured. Every man who has had occasion 

 to determine for himself the accuracy of his micrometer, by the 

 tedious process of comparing it with many others of different 

 makers, will at once appreciate the value of this work. 



For a more detailed account of what was said at the Con- 

 gress at Indianapolis, upon this subject, I must refer the mem- 

 bers to the pages of the forthcoming quarterly journal. 



OBLIQUE ILLUMINATION AND MEANS OF OBTAIN- 

 ING IT. 



BY R. HITCHCOCK. 



[Read September 2oth, 1878.) 



Fig. I, 



It is generally un- 

 derstood that the 

 appearance of an 

 object under the mi- 

 croscope depends 

 as much upon the 

 character of the 

 illumination as up- 

 on the objective. 

 Therefore, any im- 

 proved method of 

 control ling the 

 light demands at- 

 tention, and I have 

 chosen two recent 

 accessories for con- 

 sideration in this 



