ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



93 



is mounted on a stout mahogany base, provided with levelling-screws 

 and spirit-levels. The lever clockwork movement is of first-rate quality, 

 and a fine adjustment for precisely setting the position of the instru- 

 ment is afforded by a rackwork-and-pinion and tangent-screw. The 

 mirror is parallel-worked, of fine quality. 



Method of Using Abbe's Apertometer.* — F. J. Cheshire points 

 out that the method of using Abbe's apertometer with a lamp-edge, as 

 given by Dallinger,| is open to an error if the lamp is put too near, and 

 if it be assumed that the centre o' (fig. 13) of the focussing disc is also 



Fig. 13. 



the centre of the circular edge of the apertometer ; in reality. this latter 

 point is o, the middle of the chord. Let this distance o o' be d, and 

 the distance from o' to the lamp L be D. Describe a circle with o' as 

 centre, and o' L as radius. Suppose the adjustments made so that a is 

 the semi-angle as usually taken, a the true semi-angle, so that a = a -f a 

 small angle (3. Then it can be shown that the numerical aperture (as 



. , ad cos a sin a- '/."V -■~™- M 

 found) = true numerical aperture -| ~~~f) ' ■'-^ UIS "^ term- = 



— ^Fv — , and will have a maximum when a = 45°, i.e. for_N.A.'s 



* Journ. Quekett Micr. Club, Nov. 1902, pp. 349-52 (1 fig.). 

 t Dallinger-Carpenter, 8th ed., pp. 394-5. 



