682 SUMMARY OF CURRENT RESEARCHES RELATING TO 
divided scale t is fitted just beneatli the stage, and can be moved back- 
wards and forwards by a small lever H. Its central position is marked 
by a stop. 
Dovetailed in the stage of tbe Microscope is a plate W, movable 
from right to left, and carrying on the edge nearest the observer a 
division on silver s with a vernier N reading to 0*05 mm. 
The glass scale T is divided in 1/2 mm. ; the scale t, in the central 
portion of 2 mm. length, in 0*05 mm. 
The body-tube has the usual coarse- and fine-adjustments. 
The draw-tube carries a millimetre division, giving the total tube- 
length. For use with the apparatus there are five objectives, numbered 
1-5, and an eye-piece, with removable optic lens, in the diaphragm of 
which either a double cross wire or a micrometer scale (10 mm. divided 
in 0* 1 mm.) can be fitted. The objectives are screwed to the body-tube, 
and centered by means of the arrangement movable by the screw Z. 
The eye-piece simply slides into the body-tube, and is clamped in 
position by a screw. 
The height of the tube-support above the stage is 50 mm., so that 
measurement can be made of systems of this height, and with a diameter 
of 100 mm. 
As at present constructed, the apparatus serves for the measurement 
of systems of somewhat more than 80 mm. focal length, and of about 
20-100 mm. aperture. 
In making an observation with the instrument, the objective 1 (of 
shortest focal length) is first used, and the cross wire in the eye-piece 
is arranged with the double wire from front to back. The Microscope is 
then adjusted on the scale t , which is illuminated by the mirror beneath 
the stage. The objective is centered so that the middle division of the 
scale appears between the double wire in the eye-piece, and the position 
is noted in which the horizontal cross wire cuts the micrometer division. 
The movable plate of the stage is then displaced until the zero of the 
vernier points to the middle division of the scale s. 
‘ r The system S to be measured is now placed on the scale t, ad 
is moved, while the Microscope is again adjusted on the scale, until 
the same mark of the scale t appears between the double wire of the 
eye-piece, and the horizontal wire cuts the image of the division of t at 
the same height as before. 
The plate W is now moved from the central position to the right 
until a certain division of the scale t appears between the double wires of 
the eye-piece, and the Microscope is readjusted on the scale. A reading 
is then taken on the vernier of the scale s, and the distance of the division 
of t from the central division noted. 
The plate W is now displaced to the opposite side of the central 
position, to the left, until the corresponding division of the scale as 
before, only on the other side of the middle division, is adjusted in the 
eye-piece. The displacement of the scale s is again noted. 
The same processes are then repeated for the scale T, which is ren- 
dered visible by the removal of the mirror and the upper scale. The ob- 
jective 1 is in this case replaced by one of the -lower ones chosen by 
trial. 
Let 7 j 1 and y 2 denote, for the two scales respectively, the distance of 
