252 HOW TO SEE WITH THE MICROSCOPE. 



by them in the final corrections of the objective. Of 

 all the scales of podura we have yet seen, a mount was 

 shown us last summer by Mr. Herbert Spencer, which 

 was perfection itself. As an index ot the value Mr. 

 Spencer attached to this mount, I will add that he had 

 once offered one of his first class one-tenth objectives 

 in exchange for it I 



Every microscopist needs a suitable stage micrometer 

 and very few have reliable ones. We gladly state that 

 Prof. W. A. Rogers, of Cambridge, Mass., has after 

 years of careful study succeeded in making rulings on 

 glass rivaling if not excelling those of Noberl himself. 

 Not long since Prof. Rogers ruled for us a plate, con- 

 taining lines 100, 1,000, 2,000, 5,000, 10,000, 20,000, 

 40,000 and 80,000 to the inch, which was a marvel in 

 point of accuracy and delicacy. Subsequently Prof. 

 Rogers ruled a plate for us up to 120,000 lines to the 

 inch. This band the author has seen well and has 

 shown to his friends. We therefore recommend first, 

 that every student should be possessed of a micrometer ; 

 and secondly, that the same be procured from Prof. 

 Rogers. It will be well while one is about it, to order 

 a plate ruled as high as 80,000 to the inch, inasmuch as 

 the cost is not materially enhanced, the plate will thus 

 do double service not only as a stage micrometer, but 

 as a test plate for the comparison and study of objec- 

 tives. In this connection the reader will remember 

 that we have advised that eye-piece micrometers be 

 ruled 600 lines to the inch. Hence, if our advice is 

 followed, let the stage micrometer have a similar band. 



