INITIAL MAGNIFYING POWERS OF OBJECTIVES. 



297 



For lower powers the accompanying table, computed by the 

 above formula, gives the necessary tube length. 



It must be noted that it has been assumed that the screw 

 micrometer with the positive eye-piece is an English one, with 

 50 threads to the inch, but if it is a Continental one, with a 

 millimetre thread, a millimetre stage micrometer must be used, 

 and the proper number of divisions measured. If it is found that 

 the magnification is so high that two divisions cannot be spanned 

 by the micrometer webs, then obviously one division is measured 

 and the reading is doubled.* 



TABLE. 

 0, objective ; N, nominal power ; T, tube length in inches. 



Let me again impress upon microscopists to measure, or get 

 measured, the optical indices of their objectives. The optical 

 1000 N.A. 



index is 



m 



Photographers have the same thing in their 



//4, //16, etc. No photographer would think of paying as much for 

 a lens of f/lQ as he would for one, of similar focus and qualitv, of 

 fji) then why should a microscopist? A microscopist, for example, 



The foci of a large number of all sorts of microscope objectives, which 

 had been previously accurately determined by the long method, were 

 remeasured by this new short method ; the results obtained were so 

 satisfactory that now only the short method is used. 



