234 



SUMMARY OF CURRENT RESEARCHES. 



the right operates downwards, lowering the focus by depressing the 

 sleeve C carrying the body of the Microscope, and compressing the 

 spring G. The lower head B has, in this instance, simply the action 

 of a washer placed between A and C. A left-handed rotation of A, like 

 the loosening of a nut, permits the expansion of the spring to force up- 

 wards both C and B, until stopped by A. When, on the contrary, B is 

 turned to the right it screws itself into C, thereby tending to leave a 

 space between A and B. But this space is at once taken up by the 

 expansion of the spring, thereby raising the cap C together with B, and 

 consequently the focus. Thus, although both screws are right-handed, 



the milled heads produce opposite 

 motions in the adjustment. Hence 

 the simultaneous rotation of the 

 two heads in the same direction 

 will give the same effect as a dif- 

 ferential screw, provided the 

 screws are not of the same pitch. 



In another common type of in- 

 strument the limb, actuated by a 

 lever, moves in a slot cut in the 

 pillar. To this Microscope the ad- 

 justment just described cannot be 

 applied, and as the differential 

 screw is unnecessary for the pur- 

 pose, Mr. Ashe replaced it by a fine 

 micrometer screw, leaving the rest 

 intact as in fig. 40. Fig. 42 shows 

 how this design has been carried 

 out by Messrs. R. and J. Beck, who 

 have co-operated with the author 

 for the purpose. A strong, quick- 

 moving screw A. in the centre forms 

 the coarser adjustment, whilst, in 

 place of a central fine micrometer screw, a thread is cut upon the 

 exterior of a cylinder of large diameter, attached to the nulled head B, 

 thus reducing wear and tear to a vanishing point, and adding greatly to 

 its durability. In this form the upper milled head controls the quick 

 screw, which in practice proves to be a more convenient arrangement 

 than does the converse. Fig. 42 is essentially the same adjustment as 

 that adopted by Reichert in one of his latest instruments,* but it was 

 worked out independently and was completed and manufactured before 

 Keichert's instrument was heard of in England."]" 



Seibert's Mineralogical Stand.* — This stand (fig. 43) is the most 

 developed member of a set of four " Polarization-Microscopes," made by 

 the Wetzlar firm. The unusually high projection of the arm joining 

 the pillar and tube permits the examination of very thick objects with 

 weak magnification, as well as the application of rotatory apparatus on 



* See The Microscope and its Revelations, 8th edition, pp. 210-1. 

 t The idea of a two-speed fine adjustment is not new. It was worked out by 

 Anderson in 1886. See this Journal, 1886, p. 325, fig. 62.— Ed. 

 I Catalogue No. 30, pp. 28-36, fig. 12. 



Fig. 42. 



