CLOUD OBSERVATIONS IN VICTORIA. 265 
signals or clock-beats are transmitted by the Observatory according 
to a code, and the last of a set of five beats sent by hand indicates 
the instant for exposure, which is made simultaneously by the 
two observers by means of a Picard shutter worked instanta- 
neously by the usual india-rubber ball method. 
The ten pairs of photographs before you were selected amongst 
the best and the worst. They are enlargements on bromide paper 
twice the size of the negative. The heights of the respective 
clouds were determined by measuring a few corresponding points 
by a millimeter scale, and vary from 17,212 to 37,682 feet. 
The fine cirri, though sometimes barely visible on prints, can be 
measured fairly well; but the direct measures from the negatives 
are far more satisfactory. Under ordinary conditions the highest 
clouds can be measured within a probable error of 500 feet. 
The illustrations (plates XIII, XIV and XV) show two views 
of the cloud camera in use at the Melbourne Observatory, and a 
pair of photographs of alto-cumulus cloud, taken simultaneously 
from the terminal stations. 
No. 11.—THE TESTING OF REFLECTING SURFACES. 
By P. Barracui, F.R.A.S., &e. 
[ Abstract. | 
THE author, after a brief introduction on Focault’s methods of 
testing reflecting concave surfaces used in various forms of reflect- 
ing telescopes, deals with the optical theories upon which these 
methods depend, and explains, by the aid of diagrams and records 
of actual experience, how the arduous process of figuring a surface 
into the required form can be safely guided by a knowledge of the 
amount of aberration at the centre of curvature, measured in zones 
over the whole surface at brief intervals during the last stages of 
the work. This is followed by a description of the apparatus 
with which these testings and measurements are made. (The 
apparatus in actual use for this purpose at the Melbourne 
Observatory was exhibited on the table for inspection.) The paper 
then treats of the method for testing the combined effect of the 
two reflecting surfaces of the Cassegrain telescope before they are 
mounted in the tube. The arrangement of the mirrors under 
test is represented in attached diagram, in which is traced the 
path of the rays from their passage through a small diaphram 
placed in front of the artificial source of light (a small kerosene 
lamp), to their converging point at E where they enter the eye of 
the observer. 
The details of the special case of testing the two mirrors of the 
Great Melbourne Telescope, as practised at the Melbourne Obser- 
vatory, are given at length, showing the separate and combined 
