DESIGN FOB PHOTOGRAPHIC TRANSIT CIRCLE. 
213 
For reading the right ascension circle the microscope O is 
provided, and is to be of the same power as those on tlio declination 
circle, so that the observer would easily see of a second of time on 
the EA circle; S in Nos. 2*13 and 15 show the shape and massive 
character of the lower bearing of the polar axis 1VW in No. 8 ; and 
XX show the iron part of the support of the upper end of the main 
axis — it is divided to admit of collimation observations. 
In No. 4, D shows the segmental bearings of the telescope axis, 
the piece Y takes out to let the telescope axis in, and support is given 
by all three segmental bearings. The shape of Y is shown in No. 5, 
the projecting part of which takes the end thrust of the telescope axis 
when the polar axis is turned round. T No. 0 shows how the upper 
and lower bearings of the polar axis TT are attached to the girder. 
Z is a wooden table to carry chronograph and writing material for the 
observer at microscope 0. 
The graduation of the declination circles would require to be of 
the ordinary first-class quality ; that of the EA circle would have to be 
unusually fine for 15 minutes before and after the meridian in both 
positions of the polar axis, because the right ascension of the object 
would bo determined by the observer taking transits of these divisions 
under his microscope and recording them on the chronograph. 
We proceed now to adjust this instrument and take an observa- 
tion. Collimators, of course, would be required as in an ordinary 
transit circle. Wo turn the telescope until the EA circle reads the 
meridian, and proceed to collimate as in an ordinary transit instru- 
ment. Having finished this we turn the polar axis 180 degrees, and 
collimate as before. The telescope is now turned vertical, and the 
nadir and level reading found by reflection from mercury. The single 
EA wire in the telescope having been set to the collimation reading, 
the telescope is moved by means of the fine motion in EA until the 
wire covers its own reflection. A set of level readings is taken to see 
if the setting vertical has been exact ; if not, the error goes as a 
correction to the reading for the meridian at microscope O. The 
polar axis is again turned 180 degrees, and the determination of nadir 
and level is made as before. We have thus determined the collima- 
tion, nadir, and level of the instrument in both positions, as well as 
the flexure in the telescope tube and in the main axis. AYe test the 
elevation of the polar axes by observing stars in the meridian. 
We next observe stars above and below the pole in both positions 
of the polar axis, to determine the azimuth ; if it comes the same in 
both positions we know that the telescope axis is at right angles to the 
polar axis, and that if not it must be adjusted, and the polar axis 
must be also adjusted for any error of azimuth by moving the upper 
segmental hearing until it is adjusted. This motion, as well as that for 
vertical adjustment of the polar axes, is provided for in the cup-hearing 
at the other end. 
Having now adjusted the instrument as an ordinary transit, and 
in its polar axis, it will be necessary to see that the star camera line 
of collimation is parallel to that of the telescope. 
ADJUSTMENT OF CAMEEA. 
The provision for holding the photographic plate is similar to that 
used in the star camera, which holds the plate firmly against carefully 
