198 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



which was without arrangement for rotation ; and the mirror was 

 not jointed. The model of T. Ross had, as will be seen, a bar move- 

 ment, with a foot formed of a triangular plate to which were bolted 

 two parallel upright plates to carry the trunnions of the microscope. 

 The fine adjustment is a lever of the second order, with the milled 

 head in the middle of the bar, which involves tremor, and the tube of 

 the nose-piece is short, making shake possible. 



The stage movements are of unequal speed, the lateral move- 

 ment being slower than the vertical. There is no finder, and the 



rotation of the stage is 

 but partial. The sub- 

 stage and mirror are 

 good. It was a com- 

 manding instrument in 

 its day, and was of ex- 

 cellent workmanship 

 and finish ; but it was 

 not equal to the strain 

 of critical work with im- 

 mersion objectives of 

 great aperture. Never- 

 theless the defects of this 

 stand could have been 

 readily corrected. With 

 a more extended base, a 

 better arrangement of 

 the fine adjustment, a 

 mechanical stage con- 

 structed on better prin- 

 ciples, and the rotation 

 made complete and con- 

 centric which it was 

 not this would have 

 been, even for our pre- 

 sent requirements, an 

 admirable instrument. 



This important firm 

 were otherwise advised, 

 however; and, instead 

 of correcting the errors 

 of the instrument whose 



history they had made, they designed an entirely view model in which 

 a Lister limb was substituted for the bar movement. Fig. 160 illus- 

 trates this form of the instrument, from which it will be seen that the 

 foot also was changed for the worse ; the base was not sufficiently 

 extended, and the hinder part of the foot was too large, so that it 

 sometimes rocked on four points, because the hinder part was too 

 wide a flat surface, in fact. A true tripod will stand firm on an 

 uneven table, but this form will not. It is a form frequently used by 

 various makers now, and is known as the * bent claw.' It is a bad 

 design, and may be, as it has been, easily thrown over laterally. It 



FIG. 160. Ross-Zentmayer model (1878). 



