122 SUMMARY OF CURRENT RESEARCHES, ETC. 
known by observing through the reading lens attached to one micro- 
meter the inverted image of the other, coincident, but reversed in posi- 
tion (exactly as in Silbermann’s old form of focometer), The equality 
of object and image in size — known by the fitting of the micrometer 
scales — will serve to check the correctness of the observation. The 
distance from S : to S 2 =4/+ k. Hence k = 2 Fj F 2 — Sj S 2 : and/ = 
Fj S x = F 2 S 2 . By measuring off backwards from Fj a distance equal 
to S! F 2 , the point H 2 is arrived at. Similarly Hj is arrived at, and 
these points can be marked off on the outside of the tube of the 
objective.” 
Description of the Instrument . — In accordance with the foregoing 
project, the author designed an instrument which he terms a Focometer. 
It was constructed by Messrs. Nalder Bros., of Clerkenwell, to whom 
the author is indebted for many useful suggestions embodied in the 
apparatus. The construction is shown in the accompanying figures. 
The support for the lens or combination of lenses to be examined is 
fixed at the middle of a bench made of two parallel girders of gun-metal, 
each 670 mm. in length, placed vertically above one another, and 
secured together both at their ends and at the middle. The highest and 
lowest faces of this double girder are bevelled at 45°, and a scale of 
millimetres is divided along the front face at the upper edge. This 
girder frame is shown from the back in fig. 15, in end view in fig. 16. 
The support for the lens can be raised by a dovetail slide worked by 
a rack, or moved horizontally transversely to the bench by another dove- 
tail slide furnished with a screw motion, as shown in figs. 15 and 16. 
The travelling supports for the micrometers are two solid pieces of 
brass, which fit over the bevelled edges of the girders and slide without 
any looseness of motion along the frame. Each bears a vernier to read 
off' its position on the bench, and each is furnished at its upper point, 
as shown in fig. 15, with a horizontal slide for fine adjustment, worked 
by means of a screw of fine pitch ; the position of the horizontal slide 
being read off by means of a vernier against a short scale cut upon the 
face of the support. Except when the clamps described below are applied, 
each of these supports is so far free that it can be pushed along the bench 
by hand, but is fitted to slide so accurately that it cannot be shifted by 
any chance touch. 
Between the parallel girders, running from end to end of the appa- 
ratus, is a double screw, the two halves being respectively right and 
left-handed, each accurately of a pitch of two millimetres. This screw, 
the function of which is to shift the two supports for the micrometers, is 
furnished at each end with a large milled head, and with a driving 
handle. The screw is of steel. It was constructed in two separate parts, 
which were then united by being securely riveted into a short cylinder 
of steel of larger diameter. This cylinder runs through a bearing in the 
central piece of the frame of the instrument, and is secured in position 
between two fixed collars of steel, which are seen edgeways in fig. 15. 
These collars are screwed up sufficiently tightly to prevent any end 
play. At the two outer ends the screw passes through two bearings 
in the end supports, which admit of longitudinal play so as to allow for 
any difference of expansion between the screw and the frame of the 
instrument. 
