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Transactions of the Society. 
take our measurements directly along the circular arc C B D of the 
figure ; and this is the object of the instrument about to be described. 
A mere inspection of figs. 78 and 79 will indicate the nature of 
the instrument, and its use. 
A, being, as in fig. 77, the extremity of the axis of the Micro- 
scope, LMN is a graduated arc of 60°, fitted accurately upon the 
open end of the draw-tube, and secured in the position shown in the 
figure, by means of pins or studs which enter into notches cut upon 
the shoulder of the tube. It is essential that the draw-tube should 
not be able to turn ; in a binocular body this will of course be the 
case, and in all others it must be specially arranged. 
0 P is a radial arm, which, together with A R, attached to it at 
right angles, swings freely about the axis of the Microscope. At P is 
placed a projecting piece, shown separately in fig. 80, which may be 
termed the speculum ; this piece is slipped over the end of the radial 
arm 0 P, and kept in position at right angles to it by a binding 
screw. The speculum may be placed with either face uppermost ; one 
being white with a black central line, the other black with a white 
line. 
The arc M N is divided into degrees and parts (not shown in the 
figure) ; and at R there is a vernier, reading to the tenth of a part. 
To use this instrument, a camera being placed at A, any part of 
the image of an object in the Microscope can readily be brought upon 
the face of the speculum ; one edge of the image being then brought 
into contact with the line on the speculum, the arm 0 P is swung 
round until the same line coincides with the opposite edge, and the 
angle passed through is read off upon the arc M N. 
This will give the dimension of the image, with perfect accuracy, 
measured along the circular arc C B D in fig. 77, and therefore upon 
an area of uniform magnification ; the linear value of the measure- 
ment, to any radius, being ascertained at once from the table of 
circular arcs. 
It is to be noted that the instrument will give accurate measure- 
ments only in one plane ; that, namely, which cuts the axis of the 
Microscope vertically, at its point of intersection with the reflecting 
surface of the camera ; the image should therefore be so adjusted that 
the required measurements may lie along the diameter in which the 
field is intersected by this plane ; this will coincide with the horizontal 
diameter of the field, as presented to the observer looking through the 
camera. 
It is also obvious that the action of the instrument is independent 
of the inclination of the Microscope body, or of its distance from the 
table ; all that is requisite is that the radial arm shall have room to 
swing. The graduated arc may be of any dimensions, and the 
speculum may be adjusted to any length of radius to suit the 
observer’s sight. In the instrument which Mr. Holtzapfel has made 
for the writer, the arc has a radius of ten inches, and is graduated in 
