PKIXCIPAL MODERN STANDS 155 



changed the form to that shown in fig. 123, p. 158. Ross tried 

 various modifications of this fine adjustment and model, but from 

 about 1843 he worked only at the lever method as applied to the 

 nose-piece through the * cross arm ' and brought it to a relatively 

 high state of perfection. But the full possibilities of this method, 

 as concerned its sensitiveness, were never utilised by Ross, and it 

 was Hugh Powell who first published an account of his long lever 

 fine adjustment in the ' London Physiological Journal/ November 

 1843. The published account of Ross's long lever fine adjustment 

 did not appear until a month later, viz. December 1843. 



In 1835 Powell made a microscope with an extremely delicate 

 fine adjustment applied to the stage. The mechanism and the 

 workmanship were excellent (we give a drawing of a later form of 

 the instrument at fig. 121), and this fine adjustment is one of the 

 slowest and steadiest as yet made. In one we have measured the 

 movement only amounts to ^-jir f an i ncn f r one revolution of the 

 milled head ; this is six times slower than the fine adjustment applied 

 to the best Continental microscopes. The disadvantage of this fine 

 adjustment is that it slightly disturbs the focus of the sub-stage con- 

 denser; therefore, if the fine adjustment is much moved, the sub- 

 stage condenser will require refocussing. The movement usually 

 required is so slight that the refocussing of the condenser is seldom 

 required. 



James Smith also made an instrument on an entirely new 

 plan. It is illustrated in fig. 122, being the first model made 

 by this firm in this form, and it has many features of interest 

 from the point of view of our present requirements. But after 

 we have once secured steadiness, the crucial points in a microscope 

 are the quality of the fine adjustment, and the delicacy, firmness, 

 and ease with which we can centre, focus, and otherwise modify 

 the sub-stage illumination. To the former certainly this model 

 does not contribute. 



We are now prepared to examine and endeavour to judge im- 

 partially from a practical point of view the merits of the principal 

 English, Continental, and American models which are offered to 

 the microscopical public. It is impossible, no less than it is unde- 

 sirable, to attempt to describe all the microscopes of every maker, 

 or even the principal forms made by the increasing multitude of 

 opticians. We have sought no opticians' aid ; we have carefully 

 examined all the forms that lay any just claim to presenting an 

 instrument which meets the full requirements of modern microscopy ; 

 and, although we have reason to know that the judgments we express 

 are shared by the leading experts of this country, we take the sole 

 responsibility for these judgments. Having sought for tw r enty years 

 the best that could be produced in microscopes and objectives our 

 judgment is given with deliberation and wholly in the interests of 

 science. 



In examining the principal modern microscopes w r e shall point 

 out whatever is of absolute importance or relative value ; and the 

 absence or presence of this in any form provisionally selected is all 

 that the reader will need to enable him to become convinced of our 



