TOTAL SOLAE ECLIPSE OE JULY 18, 1860. 
375 
length, and divided into inches, tenths, and -^ths, the inches being numbered 0 to 4. 
On the stand is fixed the vernier a! (fig. 2), which reads to T^^th of an inch, and by 
estimation to 
On the slide A are two guide-bars, between which the slide B works at right angles 
to A; the guide-pieces are adjustable by means of set screws a"” (fig. 1), to ensure the 
rectangularity of the shde B. Slide B is moved by means of the steel rod screwed 
at one end, and carrying its clamping-bar and screws V” in the same way as the slide A. 
It has also an attached scale h, and a vernier h’, fixed to the slide A, which reads with 
the scale h to the same quantities as the vernier a' of the scale a. 
On the slide B, which is perforated, is a hollow axis, somewhat more than 4 inches in 
diameter, in which works the hollow axis of the divided circle C, which reads by means 
of its vernier V to 10" of arc. The circle C is clamped by the clamp c, and can then be 
moved by means of the milled head d, attached to a tangent screw. The vernier V and 
clamp c are fixed to the slide B. 
On the hollow axis of the circle C is a second divided circle D, which reads to minutes 
of arc, and is divided into four quadrants ; the vernier Q (fig. 1) being attached to the 
lower circle C. The circle D is only used for axial adjustments, to bring the position- 
wires depicted on the photographs to parallelism with the wires in the microscope M. 
The circle D has four dogs fastened on it, through which work the screws d, which 
carr)’ along with them four pressure plates, with two projecting wires in each, to act 
against the photographic plate P, and make it central with the instrument. The photo- 
graph rests on the four ivory studs d'. 
The photograph to be measured is in all cases placed with the collodion side down- 
wards, in order to ensure a constant distance £fom the microscope ; if placed upwards, 
any variations in the thickness of the various glass plates would necessitate a change in 
the position of the microscope at each operation. 
In the first instance it is necessary to determine that particular position of the slides 
A and B in wliich the axis of the circle C corresponds with the centre of the cross wires 
of the microscope, which is very accurately and rapidly accomplished in the following- 
manner : — A glass plate of suitable size has ruled upon it with a writing-diamond two 
lines which intersect each other as nearly as may be at right angles : this plate is placed 
face downwards on the ivory supports d\ and the slides A and B are brought approxi- 
mately to and clamped in the central position, which is at about the division 2 inches on 
their respective scales ; by means of the screws d, the cross on the glass is made to 
coincide with the cross wires of the microscope. The circle C is now turned through half 
a revolution, when the cross on the glass plate will be found to have shifted. Half this 
deviation, according to its direction, is corrected by the screws a'' and b" of the slides A 
and B, and half by the centering screws d of the circle D. After a few trials, the cross 
on the glass plate will not shift during the rotation of the circle C ; when this is the 
case, the verniers of the slides A and B are observed, and the readings noted down. 
The centre of the circle C (and consequently of D) was found to coincide with the 
cross of the microscope when A read 2-0025 inches and B 1-981 inch; and these posi- 
