ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



395 



a half -inch is required. The nominal power of a half -inch is 20, which 

 is p, then 



15 \Zid + °' 835 = 15 V " 385 = 15 x 0-62 = 9-3 in. tube-length. 



The tube must be drawn out until the web is 9*3 in. from 

 the nose-piece, and with the half-inch on the nose-piece, two y^Vir m - 

 divisions on the stage micrometer are spanned by the webs. The drum 

 then is read, say, 22 ■ 4, and this is the initial power of that half -inch, 



without any further calculation ; its focal length is k^ti or " 446 in. 



In case the nominal initial power is unknown, it is first determined 

 with, say, a 9i-in. tube ; the value thus found is inserted in the 

 equation and the measurement made again with the correct tube-length. 

 All powers of ^-in. and less focus, all Zeiss's apochromats of whatever 

 focus, and other makers' apochromats, require a 9-in. tube. 



For lower powers the accompanying table, computed by the above 

 formula, gives the necessary tube-length. 



TABLE. 

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



J. Grundy gives some experimental details of Nelson's method. 

 The Microscope is placed horizontally ; a low-power objective, 3, 2, or 

 1^ in., according to circumstances, is placed in position ; screw-micro- 

 meter eye-piece ; the objective to be measured is placed in substage, 

 with its front lens facing the stage. A card cut to the pattern as 

 shown in figure (fig. 34) is fixed by means of a clip in front of the 

 window ; the card should be placed at the exact measured distance of 

 100 in. from the stage of the Microscope. The stage micrometer is 

 placed on the stage, and the constant of the screw-micrometer deter- 

 mined. The focus of the Microscope is not to be disturbed, but, by 



2 D 2 



