ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 577 



with the concave mirror, and therefore, when used for opaque illumina- 

 tion, is practically always in focus whatever the inclination. The mirror 

 can be swung below the stage for use with transparent objects. This 

 arrangement is very convenient, as the mirror arm is not in the way of 

 the lingers when manipulating the object. 



Another point to be noticed is that the front lens and cell of the 

 -J- object-glass are coned to the utmost that is possible without limiting 

 the aperture. The apes of the cone is so small that very effective 

 illumination of opaque objects by the concave mirror can be obtained 

 even with this high power. 



When the Microscope is in use the fitting on the base-board in 

 which the spare eye-piece is packed is utilised to support a wooden 

 bracket which carries a light lamp. This arrangement allows of the 

 Microscope being slid along a table for exhibition without disturbing 

 the illumination. The outside dimensions are 5 in. by 3f in. by 9J in., 

 and the weight 4 lb. 2 oz. 



Notched Fine Adjustment for Optical Instruments. §— The firm 

 of A. Pfeiffer, Wetzlar, have designed a new form of adjustment in- 



Fig. 80. 



tended to simplify the movements of the usual types of coarse and fine 

 adjustments. Fig. 80 shows it as applied to a polarimeter. The part 

 of the apparatus acting as the fine adjustment consists of an endless 

 screw on one end of a two-armed lever ; the other end of the lever 

 terminates in a handle a, a projection on which is pressed by a clip into 

 a notch. The axis of the lever can be seen on the left .of the endless 

 screw ; this axis also provides the means for securing the fine adjust- 

 ment to the front part of the polarimeter. This part of the polari- 

 meter contains the analyser, and is rotatory about the long axis of the 

 whole instrument, and surrounds the part bearing the divided circle. 

 This inner part also bears the thread in which the endless screw works 

 through an opening in the analyser tube. It will now be understood 

 that if, by pressure, the handle a be released the fine adjustment is put 

 out of gear, and the movement of the front part serves as the coarse 

 adjustment ; the fine adjustment is then reinstated by relaxing the 

 spring, and the movement completed. 



* Central-Zeit. f. Opt. u. Mech., xxv. (1904) pp. 13-44 (1 fig.). 



