MR. RONALDS ON PHOTOGRAPHIC SELF-REGISTERING INSTRUMENTS. 117 
C. The condensing lens (or a glass plane), beyond which a brass lamp {vide 
fig. 1), or when daylight is used, a mirror is placed. 
G. The tube containing the achromatic lenses. 
O. A diaphragm. 
E. The screen. e l . The slit cut through it. 
F. The case for reception of the sliding frame ( vide H, fig. 2). 
IK. The pulley, clock, &c. {vide fig. 2.) 
PP. The two pillars of the late transit instrument at Kew. 
QQQ. Parts of the frame work and braces fixed upon PP. 
XX. Bearers clamped firmly upon PP by means of cross-bars, x 1 , x l , and 
wedges, x 2 , &c. , 
A plumb-line, not visible in the sketch, is suspended from the summit 
of the frame-work and descends to X for the purpose of ascertaining 
whether any appreciable movement of the point of suspension has oc- 
curred. I have not yet discovered any such movement. 
Fig. 9. 
R. One of the two bearers which unite the upper opposite ends of QQ 
(fig. 8). 
S. Section of the detorsion-plate, &c. s l . A rectangular plate which can 
be slid between grooved pieces fixed upon the bearers (R) and accu- 
rately adjusted by means of the screws s 2 , s 2 passing through the pieces 
s 3 , s 3 . s 4 . The graduated circular plate with its central neck descending 
through a socket in s 1 *. On s 4 a pulley is supported, which is provided 
with milled-headed nuts on its prolonged axis, serving to secure it from 
revolution at pleasure, s 5 . The silken skein attached to the pulley and 
descending through a small aperture in the neck of s 4 . 
T. The upper part of a narrow box inclosing the skein. The lower part is 
seen at T, fig. 8. 
* I propose to avail myself of Sir John Herschel’s obliging suggestion of turning, with extreme slowness 
and equability, this detorsion-plate on its centre by means of a tangent screw, moved by simple apparatus 
worked below, and whilst the effect on the brass bar may be watched by the operator, even when the boxes are 
closed. 
