oe ee of Microscopic Objectives, 159 
the gentlemen who have so kindly allowed me the use of their 
objectives, and especially to Mr. Charles Stodder, of the Boston 
Optical Works, for the large number of triplet lenses, by various 
makers, which he has placed at my disposal. 
It is hoped that the method described in this paper may be 
found of value as offering a ready and reliable method of measuring 
focal lengths, so that there may be no necessity of doubt as to the 
true focal length of any objective.—Contributions from the Physical 
Laboratory of the Massachusetts Institute of Technology. 
