46 



THE AMERICAN MONTHLY 



[March, 



produce a cut ; the diamond produced 

 a pecuHar singing sound while mov- 

 ing over the surface, which is always 

 indicative that it is working well ; and 

 finally, the lines become visible when 

 filled with fine graphite. 



There is a limit beyond which lines 

 cannot be satisfactorily filled with gra- 

 phite. It is difficult to fill lines finer 

 than about ToTro? or gooTTo of an inch. 



A most surprising result of some of 

 the experiments of Prof. Rogers, is 

 that the unaided eye can discern not 

 only single lines that cannot be seen 

 with a microscope, but that it can de- 

 tect errors which the microscope will 



A Home-made ISubstage Con- 

 denser. 



It is with me very much as it is 

 with many lovers of the microscope, 

 being situated so that I have had 

 to make many of my accessories, and 

 I contribute this description of my 

 substage condenser, believing it to 

 be an adjustment that will give ex- 

 cellent results if properly made. It 

 is for low-power objectives, and the 

 cut, fig. 13, shows the objective in po- 

 sition at A. The end B has a care- 

 fully cut thread for focussing the ob- 

 jective, operated by a milled head. 



Fig. 13.— a Home-made Substage Condf.nsek. 



not show\ Thus, he has a bar upon 

 which lines are distinctly visible to 

 the unaided eye, and, although an 

 objective of low power will show 

 them, one of high power will not. 

 But even errors or imperfections in 

 ruling which cannot be seen or meas- 

 ured with the microscope, may reveal 

 themselves to the eye by a peculiar 

 wavyness of the image. He attributes 

 the failure of the objective to show 

 the lines, as mentioned above, to the 

 inability I0 illuminate the lines with 

 light of the exact angle of incidence 

 required, and the proper angle of illu- 

 mination he thinks deserves more 

 careful attention. 



If the condenser is found to be 

 eccentric when in focus, it may be 

 rotated by the milled head, and ad- 

 justed by sliding the adapter forward 

 or back in the substage (C) to 

 adjust the focus ; but if it is well- 

 made this will seldom be necessary 

 except in work with high -powers. 

 When working with my ^inch, I use 

 my.i^-inch as condenser; when using 

 a higher power I use the ^-inch as 

 condenser, etc. 



With a Swift nose-piece I can cen- 

 ter my objective to the condenser, 

 thereby doing away with the necessity 

 of focussing by the slide. D. FoLSOM. 



Chicopee, Mass. 



