434 
PROCEEDINGS OF THE SOCIETY. 
additional conveniences of finders, and the facility of using large or small 
slides. This stage was well made, and was one of the best forms he had 
yet seen of that class. He thought, however, that it would be an advan- 
tage to have the rack-and-pinion work covered up, as far as possible, to 
exclude dust, &c., and to protect it from the hands. 
Mr. Watson exhibited and described a Microscope which his firm 
had recently made specially to meet the wants of Dr. Henri Van Heurck, 
of Antwerp (see ante , p. 399). 
Mr. Mayall said he did not propose to criticize the workmanship of 
the new Microscope, as he had not had an opportunity of examining the 
mechanism and of testing the movements. He understood, however, 
from the description already published, and from a description he had 
received, which was about to appear in a new edition of Dr. Van Heurck’ s 
work on the Microscope, that Dr. Van Heurck had supplied the specifi- 
cation of the design ; his criticism would therefore be limited to the 
design for which Dr. Van Heurck was responsible. The Microscope 
was said to be intended specially for photomicrography and high-power 
work of the most delicate kind. With these aims in view, he must at 
once express his objection to the fine-adjustment, which was on what 
was generally known as the Zentmayer system, and which had long been 
condemned by those who had had special experience in testing fine-adjust- 
ments, such as were applied to Microscopes of the highest class. In the 
Zentmayer fine-adjustment, the bearings extend generally from end to 
end of the Jackson limb, and in order to secure a sensitive motion portions 
of the contact surfaces were removed to reduce the friction, and in all 
cases these bearings had to be left somewhat free, otherwise the motion 
would either be completely stopped or more or less irregular. Probably 
no one had made the mechanism more carefully than the late J oseph Zent- 
mayer, in the large Microscope for which he was awarded a gold medal 
at the Philadelphia Exhibition, 1876, and a silver medal at the Paris 
Exhibition, 1878, which instrument he (Mr. Mayall) had had in use 
during several months. He could affirm that, in spite of Zentmayer’s ex- 
cellent workmanship, the fine-adjustment was unsatisfactory, and this he 
attributed to the fact that the system involved that the coarse-adjustment 
and body-tube were carried on the delicate bearings of the fine-adjust- 
ment, and the strain thus put upon these bearings soon introduced 
shakiness that was practically intolerable in high-power work. The 
apparent simplicity and consequent economy of the system had induced 
many opticians to adopt it, with more or less modification, both in 
England and in America ; but, so far as he was aware, no permanent 
success had been attained — the radical defects of the system always 
cropped up sooner or later. He had witnessed a large number of experi- 
ments with the system conducted by Eoss and Co., who acquired Zent- 
mayer’s patent rights in England, and who had spared no expense with 
a view to rendering it as perfect as possible, but the results were not 
satisfactory, and other systems had since been substituted by that firm. 
The adoption of the Zentmayer fine-adjustment by Dr. Van Heurck 
seemed to him a radical error in the specification, especially in an in- 
strument intended for high-power work. It should be noted that other 
systems of fine-adjustment had been worked out, which were applicable 
