1895 THE MICROSCOPE. 27 



welfare, but the public office in America is now "a private snap" 

 and the incumbents think how to make money out of it. 

 Their actions mostly correspond with Vanderbilt's epiojram, — 

 " The public be d — d," and it often is — with disease and death. 



QUESTIONS ANSWERED, 



NoTB. — Dr. S. G. Shanks, of Albany, N. V , kiniHv consents to receive all sorts of ques- 

 tions relating to microscopy , wh- eke usk-d by f> o/essionals or am.U. u- s. Persons oj ui^ 

 f ades of experience, from tke begi n^y upwa-d. ii>e7o Line to the ben, Jits /this dr-pw' 

 ment. The questions are number id /or •unn e ■ eje> e>.ce. 



218. Please inform me how to measure the value,, or magnifying 

 power of objectives and eye pieces? — Dalton. 



Col. J. J. Woodward's method is not very precise but simple 

 and sufficient for ordinary purposes. Use an eye-piece fitted 

 with a micrometer, but with the field lens removed. Place a 

 stage micrometer beneath the objective. When in focus, the 

 two micrometers must be at least 10 inches apart. Then the 

 true focus of the objective will equal the magnification multi- 

 plied by the distance and divided by the square of the magni- 

 fication plus one. Suppose when adjusted as above, that one 

 division of the 1-100 inch on the stage micrometer, at 10 inches 

 distance, appears to cover 18 divisions of 1-100 -inch, of the eye- 

 piece micrometer then 18 multiplied by 10 and divided by the 

 square of 18 plus 1 ; equals 180 divided by 361 ; equals .5 or 

 one-half inch, the focus of the objective. If one uses a positive 

 eye-piece, in which the micrometer lies below the field lens, the 

 latter need not be removed. See A. M. M. J. 1885, page 141, 

 for more accurate methods for finding the optical center and 

 rating of an objective. 



The following approximate method will enable one to deter- 

 mine the magnifying power of an ocular. Use an objective of 

 known focus, i. e. power, and a tube length that will place the 

 optical centers of the eye-piece and objective ten inches apart, 

 as nearly as possible. Place a stage micrometer in position, 

 turn the microscope body to the horizontal and place the in- 

 strument on a box or on blocks to raise the center of the eye- 

 lens just ten inchps above the table. Slip on a camera lucida 

 and compare the image of the micrometer ruling with an inch 



