A BOSS'S ' LISTER ' MODEL 



153 



The stage is actuated in diagonal directions on either side of the 

 stem. A Pritchard microscope probably made by Ross is shown in 

 fig. 119. It is not at all like fig. 118. The stage movement is 

 by rack and pinion and not by screw as in fig. 118, but it will 

 be seen that it has also a curious spiral fine adjustment, which is 

 plainly an uncovered ' Continental ' form, either adopted in England 

 from G. Oberhauser, or it may 

 have even preceded it. It is 

 interesting to note, however, 

 that the sub-stage arrange- 

 ments in both these instances 

 are the same as those employed 

 by Wollaston in connection 

 with his celebrated doublets, 

 an account of which was given 

 in the Philosophical Transac- 

 tions of that date. 1 



The Ross form cannot be 

 inclined, nor can the Prit- 

 chard; and 'the fine adjust- 

 ment in the former is effected 

 by means of a long screw 

 passing up the pillar and act- 

 ing on a triangular sheath, 

 within which the stem is 

 applied, to move with rack and 

 pinion, the top of the stem 

 being hollow to receive either 

 the cross-arm support for the 

 single lens or the limb of the 

 compound body. The screw 

 is actuated by a large, gradu- 

 ated, milled head below the 

 tripod.' 



The stage has supports 

 evidently to enable dissection 

 to be effected without flexure 

 by the weight or pressure of 

 the hands, which makes it 

 clear that it is the Valentine 

 microscope that is referred to, 

 as may be seen by reference 

 to fig. 118. Rectangular me- 

 chanical movements are employed acting diagonally on either side of 

 the stein by rather fine screws, so that the motions are slow. 



But A. Ross at an early period worked out a * Lister ' form of 

 microscope, with the limb supporting the body-tube. He applied a 

 fine adjustment in this to act upon the nose-piece only, which, as 

 we shall subsequently see, is a very inferior method. This instru- 

 ment dates from 1839, and is shown in fig. 120. In 1842 he 

 1 Trans. Boy. Soc. 1829. 



FIG. 120. A Ross microscope (1839). 



