ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
507 
adjusting objectives of short focal length and immersion systems, which 
must otherwise be done by testing, and consequently with danger to the 
apparatus. In determining the refractive index of a transparent plane 
parallel plate, Herr Kayser proceeds as follows. The refractive index 
is = — — where D and d are given by three readings on the scale, 
when the adjustment of the Microscope is made : — 1. On the support of 
the plate. 2. On the upper face of the plate, after it has been put on 
the stage. 3. On the support as seen through the plate. The readings 
1 and 2 give D, the readings 1 and 3 d. This method is, however, not 
sufficiently precise. Another and more exact method, with experimental 
proof, will be given later. 
(3) Illuminating and other Apparatus. 
Mayall’s “ Jewelled” Fine-adjustment. — At the April meeting of 
the Society, Mr. J. Mayall, junr., referred to an improved form of fine- 
adjustment constructed and exhibited by Messrs. Powell and Lealand, 
for the production of which he was himself chiefly responsible. He said 
that during the past ten or twelve years, several forms of fine-adjustment 
had been brought to the notice of the Society, but the principal aim in 
most of them had been economy of production or lowness of price, 
without regard to improving on the best existing forms. In the new 
form exhibited the chief aim had been to construct a fine-adjustment 
that should combine extreme sensitiveness of action with accuracy 
and probable durability, beyond what had previously been attained. 
With this view he had carefully considered every known form of fine- 
adjustment, and had selected that of Messrs. Powell and Lealand, as 
representing the highest type of construction yet devised, with which 
to test the possibility of improvement. The essential feature in the 
improvement was the application of what watchmakers would term a 
“jewelled movement.” The whole of the contact surfaces by which 
the fine-adjustment was actuated consisted of polished steel and agate, 
the intention being to reduce the friction as much as was consistent 
with steadiness of motion. The perfection and durability of jewelled 
mechanism was a great feature in the highest class of clocks and 
watches ; the most delicate parts of Nobert’s ruling machine were 
jewelled, as were also the bearings in Dr. Hugo Schroder’s feeling 
level for testing the accuracy of plane surfaces. Those who were 
familiar with Powell and Lealand’s fine-adjustment, as previously con- 
structed, would understand the extreme difficulty of improving the 
mechanism substantially, for it was the outgrowth of long experience 
and of the most conscientious devotion of expert mechanicians to the 
task of providing a perfect focusing movement. No other fine-adjust- 
ment had reached the same high standard of construction, which was 
probably due to the fact that during the fifty years that had elapsed 
since its first production, the makers had kept steadily to the same 
system, only varying the minor details of the mechanism as experience 
critically suggested in the direction of improvement. 
The application of polished steel and agate bearings throughout the 
mechanism was intended to reduce the friction, and thus render the 
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