50 THE MICROSCOPE. 



milled-heads at D ; and a more delicate adjustment of this optical parfc 

 is effected by the milled-head E. The other milled-head fixes the arm 

 K to the triangular bar. H is the mirror, which slides up or down the 

 tube to which it is attached ; standing near which is the condenser 

 M, with its upright support of brass for holding the condensing-lens 

 L, this is made to unscrew, for the convenience of packing. Such 

 instruments are manufactured by Mr. Ross at prices varying from 10 

 to 100Z. 



Mr. Ross's small achromatic compound microscope is supported on 

 a firm tripod base, from which rise two strong uprights, supporting at 

 their upper parts the trunnions to which the square frame, carrying the 

 stage and triangular bar, with the body, are attached. Within the tube 

 a smaller one is made to slide up and down by rack and pinion : this 

 forms the coarse adjustment. The fine adjustment consists of a conical- 

 pointed steel screw pressing against the top of a slit in the inner tube, 

 to the end of which the adapter for receiving the object-glasses is fixed. 

 The stage has the usual rectangular motions, that from the side being 

 performed by a screw and nut, by turning the milled-head ; whilst the 

 up-and-down movement is performed by a rack and pinion, turned by 

 the milled-head below the other. The stage-plate is provided with a 

 sliding-rest, by which the distance of an object from the central hole 

 in the plate may be regulated before focussing : this is sometimes made 

 to answer the purpose of the complicated sliding frame in the more 

 expensive instruments. At the upper part of this stage-plate there are 

 two holes for the reception of the forceps and side reflector. To the 

 under part of the stage, the achromatic condensers, the diaphragm- 

 plate, and polarising-prism may all be adapted as in the larger instru- 

 ments ; and for convenience of package, the stage itself may b,e turned 

 on a pivot, so as to be at right angles with the tube. The mirror is 

 mounted in the usual manner, and is made to slide up or down the 

 tube on which it is supported. This is recommended to those whose 

 means are limited, in consequence of the low price ; it being of a form 

 which may be added to from time to time, according to the wants of 

 the employer : thus, for instance, a vertical stand, with two eye-pieces, 

 exclusive of the object-glasses, may be procured without the stage 

 movements or the fine adjustment, at the small cost of 41. 10s. ; and 

 as both the stage and the compound body are of the same size as in the 

 more perfect instrument, the fine adjustment and the stage movement 

 may be added to the former at any time, and render it as complete. 



A smaller compound achromatic microscope, fig. 37, is particularly 

 adapted for students : this is packed into a neat mahogany case, com- 



