On the Meridian Instruments of the Dudley Observatory. 407 
the avoidance of any prejudical effect arising from unsymmetric 
distribution of weight. The measurement of flexure may take 
place without disturbing the adjustments or parts of the instru- 
ment, by some apparatus analogous to the neat and practical 
contrivance of Professor Challis, who arranges a pair of collima- 
tors in such a manner that they are used in connection with each 
other at any desired angle of altitude. Moreover, the reversal 
of the instrument admits of a scrutiny and check upon the de- 
terminations, which provides all needful safeguard against erro- 
neous results. 
The circles are divided to 2’, and read by microscope to 0’"1. 
The unit’s place of the degree is always visible within the field 
of the microscope, and the decades of degrees are engraved 
Upon the rim of the circle. The finders read to 10’, and by ver- 
hier to 1’ 
The axis is turned within as well as without, a peeeenig upon 
e cube meas- 
The weight of the axis is about 350 pounds. 
, +he illumination is entirely by gas the light designed for the 
illumination of the field entering by one pivot, and that for the - 
threads by the other. Arrangements are made for illuminating 
with lights of different colors, and for observing, when occasion 
Tequires, with bright threads upon a bright field. The levels are 
oxed, provided with air-chambers, and read from end to end, 
hot from the middle outwards. : : ie 
In fine, it has been my endeavor to incorporate in the pe i 
* 
| 4 : 
ception, should be capable of determination by two sohirely dis- 
Tay al eri sses, free from any 
dependence, direct or indi upon each other. And whatever 
taay be its errors of construction or of mounting, there is no fear 
that they will escape detection and accurate measurement. 
* 
