SCIENTIFIC SUMMARY. 
307 
the auspices of the R. S. Committee has resulted in absolute failure. We 
hope (but, judging from past experience, we scarcely expect) that the lesson 
which this result ought to teach will be learned and remembered. 
Photography in the Transit of Venus . — A discussion has arisen as to the 
best way of photographing Venus in transit. In reply to objections raised 
by Capt. Abney to the American or long-focal system, Mr. Proctor, in the 
Notices of the Astronomical Society for April, makes the following remarks : 
11 The American Transit Committee, after many experiments and long in- 
quiry, came to the conclusion that the diameter of the Sun, as depicted by 
the photoheliograph, could not be ascertained (at least with the extreme 
accuracy necessary for determining the solar parallax) either by calculation 
(depending on the optical adjustments), or by direct measurement, or by any 
practical contrivance, such as photographing a scale. On the other hand, 
they consider that reliance can be placed on the calculated scale of a picture 
taken at the principal focus, while the centres of the discs of Venus and 
the Sun can be determined accurately, because each centring results, not 
from a single pair of measures, but from as many all round each limb as the 
observer may wish to make. Of course the position of the centres may be 
determined in the same way, from a photoheliographic picture ; but no ad- 
vantage results if there is no trustworthy scale of measurement. This, then, 
is the point at issue, viz. whether a trustworthy scale can be obtained. 
The available methods are (i) calculation, (ii) measurement of the pictured 
solar diameter, and (iii) photographing a scale. As to the first, I apprehend 
that there can be no comparison in point of exactness between the calculated 
scale of a picture at a principal focus and that of a picture optically enlarged : 
it is only necessary to consider the optical adjustments and relations in- 
volved, to be assured of this. As to the second method, it matters little 
whether photographic irradiation be large or small ; for at the lowest estimate 
ever yet made, the effects of irradiation must be fatal in such a problem as 
determining the solar parallax : apart from this, we now have evidence 
showing that the photographic sun is really larger than the optical sun. 
As to the third method, it seems sufficient to note that the use of a photo- 
graphed scale introduces of itself a probable error as large as that in single 
measurements of the photographed disc of the Sun or Venus ; and such an 
error would be fatal in a problem of this kind. The fact pointed out by 
Capt. Abney that daguerreotypes ‘ are subject to much greater fluctuations 
of expansion than are glass negatives,’ seems to counterbalance the superi- 
ority claimed for them (by Sir G. Airy at the January meeting) in point of 
definition. It may be hoped that before the Transit of 1882 photographers 
and astronomers will have decided on some one method of photographing 
Venus in transit. The qualities of the various methods employed on the 
present occasion will be sufficiently indicated during the examination of the 
complete series of transit observations. One point seems already clear, viz. 
that contacts determined from photographic records differ from contacts 
observed with the telescope, the photographic sun being larger than the 
sun we see. Hence greater reliance will probably be placed on mid-transit 
photographs. This country is afforded a noble opportunity of serving science 
by providing Southern stations for this purpose in 1882 — an opportunity of 
which it may be hoped that she will avail herself.” 
