66 THE MICROSCOPE AND ITS ACCESSORIES. 



seen in the figure is fitted with an x\bbe condenser. The swing- 

 ing double mirror slides in a slot cut in the bar, and can easily 

 be removed when required. 



Messrs. Swift also supply a Petrological Microscope, built upon 

 the same lines as their x^dvanced Students' Stand. It has a 

 revolving glass stage with a divided edge. xA.U the necessary 

 polarising apparatus is attached. 



W. Watson and Sons. — The " Edinburgh " Students' Micro- 

 scopes of these makers represent a good class of working instru- 

 ments. Fig. 15 illustrates their " Edinburgh " C model. The 

 instrument is supported by a single pillar, arising from a horse-shoe 

 foot. It has a rack and pinion coarse adjustment, a lever fine 

 adjustment, and a draw-tube marked for the various objectives now 

 in use. The sub-stage is worked by a rack and pinion movement. 

 It is provided with screws for centring purposes and is mounted on 

 a pivot, so that it may be turned on one side when direct light 

 from the mirror is wanted. 



The A and B stands of the Edinburgh series are m.uch simpler 

 than the C model, as, for instance, the A stand has a sliding-tube 

 instead of rack and pinion coarse adjustment ; it is also without 

 the sub-stage, as represented in C. The B stand has a rack and 

 pinion coarse adjustment, but is without the sub-stage. The D 

 has, in addition to the movements of the C stand, a complete 

 mechanical stage. 



But as the Tripod form of foot gives a maximum of steadiness 

 to the instrument, and Dr. Dallinger, in his new edition of 

 " Carpenter on the Microscope," says : — " A broad base, resting 

 on three points only, and these blocked with cork, is the ideal 

 for a perfect instrument " ; Messrs. Watson and Sons have con- 

 structed an instrument to meet these requirements. Fig. 16 

 shows Stand H of the Edinburgh Students' Microscope, made by 

 Messrs. Watson and Sons. The original of this instrument was, 

 as shown in Fig. 15, with the continental horseshoe base and 

 pillar. The plainest of this series of instrument has only a sliding 

 fitting for the coarse adjustment of the body, and an under-fitting 

 to carry the Substage Condenser, so that a student buying only 

 the plainest form of instrument can have the most perfect form of 

 foot. 



