H. F. ANGUS ON THE APERTOMETER AND ITS USE. 21 1 



aperture becomes coincident with the optic axis of the microscope ; 

 then, on removing the eyepiece and looking down the tube, the 

 whole of that part of the protractor which tlie objective takes in 

 will be found to be visible, and the pointers can be adjusted so as 

 to touch the opposite edges of the field, the reading indicated 

 being the aperture required. Except with very low powers, 

 however, the image of the protractor seen when looking down the 

 tube is so small that some difficulty will be experienced in 

 adjusting the pointers. To obviate this the draw-tube can be 

 converted into an auxiliary microscope, thus magnifying the 

 original image. This is effected by replacing the eyepiece and 

 screwing into the lower end a very low-power objective, such as 

 the posterior half of a 2 -in. objective (the most suitable power 

 varies somewhat with the available tube-length of the microscope). 

 Care should be taken, when unscrewing and replacing the draw- 

 tube, that the focus of the objective under examination is not 

 altered in any way ; and, when the auxiliary microscope is in 

 position, any requisite focussing should of course be effected by 

 sliding the draw-tube in or out as the case may be, not by altering 

 the body-tube of the microscope. 



Having in this way obtained a reading of the actual angle 

 embraced by the objective, it can be converted to N.A., either 

 by reference to some table such as that given in Dallinger's 

 " Carpenter," or to a table of sines. With care, and with 

 suitable illumination, N.A. 0*95 can be read with this instrument. 



The Suhstage-SGcde cqyertometer was suggested to me by Mr. F. J. 

 Cheshire, of the Birkbeck Institute, and consists essentially of a 

 glass disc, of such diameter that it can be dropped into the stop- 

 carrier of the condenser with which it is used, ruled with equidistant 

 lines (a millimetre scale will be found as suitable as any). To use 

 this piece of apparatus it is first necessary to find the value of the 

 scale when used with any given condenser. This is effected by 

 means of an objective of known aperture in the following manner : 

 the condenser and objective having been focussed on an object, 



