New Methods in Microscope Work. By E. M. Nelson. 145 



the small dial seen at the top of the instrument at right angles to the 

 drum, but in fig. 35 they are shown on the engraved plate -which 

 traverses across a fixed arrow-head. 



Having described the instrument we will now pass on to its 

 application for microscopical purposes. In the Journal for 1901, 

 p. 243, it was pointed out that a dynamometer would be a very con- 

 venient, if costly, instrument for the determination of the Working 

 Ratio or Aperture, and, on account of the expense, an alternative 

 device was recommended. Experience has since shown that, while 

 this alternative apparatus is quite suitable for the determination of 

 the Working Ratio, it fails to measure the precise diameter of the 

 back lens of the objective, because of the difficulty of finding the 

 exact point when the back lens is in focus. But, by measuring with 

 a dynamometer the diameter of the eye-discs, representing both the 

 full aperture, and also that portion of it which is illuminated, not 

 only can the Working Ratio be found, but the N.A. be determined 

 as well. 



In order to measure any interval by a dynamometer the constant 

 of the instrument must first be found out : in other words, a tenth of 

 an inch, or one millimetre, on a well divided scale must be measured, 

 and all other measurements referred by proportion to this standard ; 

 thus, in one of the instruments in fig. 34 one-tenth of an inch re- 



35 



quires five complete revolutions and =-^- of a revolution, which may 



be written as 5 ■ 35 ; then, if some other interval measured 3 ' 8 



3*8 * O'l 

 revolutions, it would represent — - — or 0"071 inch. 



o " oo 



It should be mentioned that these instruments are supposed to 

 read off in inches directly, but although several have been examined 

 only a single one gave a correct reading ; they mostly read too high. 

 For instance, the drum-head in fig. 34 is divided into twenty primary 

 divisions, so that the real reading in the observation above is not 

 5*35, but the double of this, or 10*70, which ought to have been 

 10 • 00, if the instrument indicated correctly. 



By far the better plan is to disregard these readings altogether, 

 and to find the constant of the instrument once for all, and apply it 

 in the manner pointed out above. It will be noticed that in carrying 

 out this idea, the drum-head in fig. 34 is read decimally, and not as it 

 is engraved; thus 14 is read 7, 16 read 8 ; in brief, the reading on 

 the drum-head is halved, and the complete revolutions left as they 

 really are. The drum-head in fig. 35 is divided decimally, and one- 

 tenth of an inch reads exactly five revolutions, or 500. The reading 

 of these instruments is affected by myopia and presbyopia, therefore 

 every observer should find out his own constant of the instrument. 



If the Working Ratio be required the two dynamometer drum- 

 bead readings need not be converted into linear measure, as their 



April 16th, 1902 l 



