DESIGN EOR PHOTOGRAPHIC TRANSIT CIRCLE. 
211 
apt to materially influence definition. It is not, I believe, an extremely 
high nor abnormally low temperature an observer need fear, but 
unsteady and constantly changing air currents, within the observatory 
as well as without, that are inimical to good telescopic work. Once 
modify or minimise these conflicting elements, maintain uniformity of 
temperature in the observatory, and the results of astronomical 
observations will probably rise 50 per cent, nearer the truth than at 
present. 
In conclusion, I have great pleasure in acknowledging the efficient 
aid rendered by the three gentlemen with whom I was associated in 
this work, and of testifying to the satisfactory degree of accuracy of 
the times recorded by them. 
8.— DESIGN FOE A PHOTGGEAPHIC TEANSIT CIECLE. 
By H. C. RUSSELL , B.A., C.M.G . , F.R.S . , Government Astronomer , New South Wales. 
The idea of making an instrument to record by means of photo- 
graphy the meridian passage of stars is not a new one, and several 
instruments have been designed for the purpose. All that I have seen 
described are attachments to the ordinary transit circle, and they seem 
to me to fall short of what is required from an instrument made for 
this purpose, because with such an arrangement it would be impossible 
to record faint stars, and the means for determining the right ascension 
are not so satisfactory as we have a right to expect in an instrument 
designed to avoid the personal equation and other sources of error 
which are inseparable from the present transit circle. In the use of 
our star camera, a description of which has been already published 
(No. 81 of the records of Sydney Observatory), I have been surprised 
at the stability, ease in manipulation, and accuracy that it is possible 
to attain, and the conviction has grown upon me that an instrument of 
the same general design, and made with all needful care, would be 
capable of recording on photographic plates the EA and NPD of 
stars with much greater accuracy than it is possible to secure with the 
existing transit circle, I therefore propose to put before you a 
design for an instrument intended to secure this much desired possi- 
bility of increased accuracy in determining the fundamental positions 
of stars. 
I am aware that such a proposition may seem absurd to one not 
familiar with photographic methods and instruments, hut the result 
proposed to he obtained is well worth discussing, and I will therefore 
submit the design for discussion, first giving a working drawing and 
description of the instrument itself. I shall then point out the 
method of working it, and the qualities which seem to me to promise 
greater accuracy than can now be obtained with the transit circle. 
THE INSTEUMENT. 
The accompanying print is a working drawing made to scale, and 
shows at once that the instrument is virtually a photographic equa- 
torial, with the telescope mounted in the polar axis. The telescope and 
camera are combined in one rectangular tube BE, Nos. 1 and 15, carried 
by an axis CC in Nos. 15 and 11 ; this is carried by the divided polar 
