1 66 THE HLSTOKY AND DEVELOPMENT OF THE MICKOSCOPE 



we hope that it may become the general fine adjustment for this 

 form of microscope in place of the old form of direct-acting screw. 



In contrast and comparison with Campbell's differential screu 

 we may put the principle on which the usual simplified construction 

 of the fine adjustment of the Zeiss stands rests. 1 In fig. 127 the 

 triangular bar C is screwed firmly to the stage ; on it moves a hollow 

 piece B, which is connected inseparably with the arm A carrying the 

 tube. At its upper end C is cut away for about 15 mm. and B 

 hollowed out at a corresponding place so that space is obtained for :\ 

 spiral spring. This spring bears below against the hollowed-out 

 part of B, its upper end being connected with the projections of the 

 piece E screwed into C. The piece B is closed above by the cap F, in 

 which is the female screw. On the top of the micrometer screw i> 

 fitted a bell-shaped head, and at its lower end is a small nut for 

 preventing over-screwing. The lower end of the screw is rounded off 

 and bears against the flat surface of a hard steel cylinder let into E. . 



Clearly, when worked, the screw remains in the same place, 

 bearing against C. The female screw, on the other hand. UK ACS over 

 it, raising and lowering the tube carrier B A connected with it. By 

 its own weight A B counteracts the rise and thus supplies the place 

 of the strong spiral spring formerly employed. The weak spring 

 here adopted acts in the same direction as the weight of A B, and 

 serves to assist the latter when the upper part of the microscope is 

 placed horizontally. 



Our appreciation of all that is done by the great firm of Zeiss we 

 need not reiterate ; it is well known ; but our opinion of the form of 

 stand adopted by these opticians we freely expressed, and we believe 

 justified in the last edition of this book ; but it is well to get the 

 opinion of one who with practical knowledge would certainly not be 

 prejudiced against the Continental stand. Dr. H. E. Hildebrand 

 says 2 that in teaching establishments, where as many as two 

 hundred microscopes may he used, the weak points of the Continental 

 stand are soon brought to light. The fine adjustment screw soon 

 becomes unsteady (an inevitable consequence of the weight so fine a 

 screw has to carry), the prism suffers bending or rotation, the prism 

 flange or the hinge-block under the object stage loosens its connec- 

 tion with the stage plate, tfcc. &c., all of which and much more, as we 

 believe, is the result of the adaptation of a simple and primitive form 

 to complex appliances for which it was never designed or intended. 



It is, however, an admirable characteristic of the firm of Zeiss, 

 that while they adhere doggedly to the old Continental model, they 

 are continuously putting forth their ingenuity and skill to counter- 

 act what are shown to be its defects. In their best usual form the 

 speed of the fine adjustment is T ^ T inch for each revolution of the 

 milled head. This is undoubtedly too rapid, but it could scarcely be 

 made a finer screw, because, as we have seen, it had the coarse 

 adjustment and tube to lift, and the wear and tear on so fine 

 a thread in constant use led to rapid failure. But the firm has 



1 This form was introduced in 1886, and was a great improvement on its pre- 

 decessor, which was mechanically bad. Vide E.M.S.J. 1886, p. 1051. 



2 Zeitschr. f. wiss. Mikr. xii. (1895) pp. 145-54. 



