132 HOW TO SEE WITH THE MICROSCOPE. 



The method has its drawbacks. For instance, all 

 microscopes do not have tubes precisely ten inches in 

 length, especially when the objective is placed in posi- 

 tion; and then, again, if the problem be to determine 

 which of two lenses is the stronger, it may occur, and 

 pretty surely too, that one glass will have a shorter 

 setting i. e., "mount," than the other. There are ways 

 to dodge the difficulty, but the best method, we have 

 seen of measuring objectives, where strict mathematical 

 accuracy is not a vital consideration, is by the use of a 

 formula, modified by Col. J. J. Woodward, and by him 

 given to the public through the columns of Silliman's 

 Journal not long ago (Vol. III., June, 1872): 



Formula : 



ML 



where F focal length, M magnification without 

 field-glass of eye-piece, and L length between stage 

 micrometer, and micrometer in eye-piece. 



If the "B" (one-inch ocular with its field-lens re- 

 moved, be selected, a micrometer ruled to one six-hun- 

 dredth of an inch will be found to suit it very well. 



It will be seen that the formula is extremely simple, 

 and quite within the comprehension of all ; we, how- 

 ever, append an example or two, selecting cases which 

 have occurred in actual practice. 



* (1.) We lately had in hand a glass of Mr. Gundlach r s, 

 made for a half-inch, and we desired to be assured as to 

 its true rating. Placing a suitable micrometer on the 

 stage, and selecting a one-inch ocular, with its field- 

 glass removed, but fitted with micrometer ruled to one 



