252 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
struction. Tlio action of the latter, it is claimed, is direct and perfectly 
steady with the highest amplification. 
The stage is large and of the horse-shoe form ; below it is a fitting 
for receiving all illuminating apparatus. 
The stand is a modified form of Powell and Lealand’s tripod. 
In fig. 18 the Microscope is represented in the horizontal position, 
with the addition of a substage provided with universal adjustments for 
centering and focusing the illuminating apparatus. The special form 
of the tripod-stand renders the Microscope very steady when used in 
the horizontal position for photomicrographical purposes ; for, as seen 
in the figure, the pillar of the fine-adjustment rests squarely on the top 
of the tripod, and the centre of gravity falls well within the triangle 
formed by the supports. 
New Substage.* — Mr. G. Whitfield Brown, jun., describes a sub- 
stage made by Zentmayer, of Philadelphia, according to his specifications, 
which “ has given perfect satisfaction.” 
The substage consists of a double elbow with two arms, each of 
which contains a tubular holder for receiving accessory apparatus. A 
centering set-screw secures it to a bracket which slides or is moved by 
rack and pinion on the tail-piece. Between the upper and lower holders 
is a removable iris diaphragm. The lower arm of the substage is pro- 
vided with a revolving plate, upon which there is a sliding plate moved 
by rack and pinion. In the central opening of this sliding plate the 
iris diaphragm fits by means of a flange, and can be readily inserted and 
removed. For the iris diaphragm may be substituted revolving receivers 
for the reception of selenites, mica-plates, diaphragms, &c. 
In this substage the Abbe condenser is fitted into the upper holder 
from above. 
3X Illuminating- and other Apparatus. 
Production of Exact Micrometer-Screws. | — Dr. Hugo Schroeder 
gives a detailed description of the mode of production of exact screws. 
A short account of the process employed will be found in the Encyclo- 
paedia Britannica, 9th edition, Part 83, p. 552. 
The principle of the method consists in first preparing as exact a 
screw as possible by the best of the ordinary methods, and then in 
eliminating the errors by systematic grinding and polishing. 
For this purpose a nut (fig. 19) is prepared of brass, or better, of 
Bessemer steel, the length of which is to that of the screw as 11 to 9. 
The nut is made up of four segments a (fig. 20), which are fastened 
together by means of the collars b b, the rings d d , and the screws c c, so 
that during the polishing the screw and the nut may be always kept 
in contact. The long nut is brought slowly over the screw and moved 
backwards and forwards over it after oil and emery have been intro- 
duced. Towards the end of the grinding finer emery is used, and finally 
oxide of iron only. 
Copies of a perfect screw thus prepared can be made by the following 
process : — The normal screw is set in the support instead of the ordinary 
* Amer. Mier. Journ., xiv. (1893) pp. 347-50 (2 figs.), 
t Zeitschr. f. Instrumentenk., xiii. (1893) pp. 217-29/ 
