420 
rorULAll SCIENCE EEYIEW. 
chord traversed by Venus for the determination — not of the real length of 
that chord — but of the greatest approach of Venus to the sun’s centre. And 
the reason for the change is obvious. If an observer were sent out to de- 
termine how near Venus approached the sun’s centre, as seen from a northern 
or southern station, he would be subject to a number of difficulties. In 
fact, a very slight consideration of the subject shows that the micrometrical 
determination of the distance would be practically valueless. But the 
photographer can at once secure a picture of the sun with Venus on his disc 
at the moment of estimated nearest approach, besides several photographs 
taken (at short intervals) before and after that moment, and the examina- 
tion of these photographs afterwards by an astronomer in his study, with 
the simple appliances of dividers and protractors, will tell everything that 
could be learned from trustworthy micrometrical measurements, were such 
measurements possible. 
There are, however, it must be admitted, difficulties of some importance 
in the application of this method. The optical considerations involved are 
of themselves sufficient to render the interpretation of the photographs a 
matter of considerable complexity. And then, again, the distortion result- 
ing from the shiinkage of the collodion film may be much more considerable 
under the special circumstances of such photographic work as we are con- 
sidering than under any of the ordinary processes. 
As Major Tennant remarks, the probable value of photographic records of 
the transit is very great, but evidence has still to be adduced on the subject 
before the method can be unreservedly trusted. 
It may be remarked tliat if it is intended to apply photography to the 
approaching transit, the places best suited for the purpose would be different 
from those available for either Halley’s method or Delisle’s. We know that 
for the former method places must be chosen where the whole transit is 
visible, and where, subject to this condition, the displacement of Venus’s 
chord of transit may be greatest either northwards or southwards. For the 
photographic method, the latter point alone need be attended to, and the 
complication, arising from the necessity of considering the earth’s rotation, 
is for the most part got rid of. Delisle’s method really involves the deter- 
mination of Venus’s displacement at right angles to the sun’s limb at ingress 
or egress, this displacement being due to the separation of the observers who 
observe either (i) accelerated and retarded ingress, or (ii) accelerated and 
retarded egress. Now nothing would be gained by placing photographic 
stations near those regions suitable for the application of Delisle’s method, 
becau.se tlie very object of Delislo’s metliod is the securing of non-simul- 
taneous observution.s, wliile the perfection of the pliotographic method con- 
sists in tlie comparison of simultaneous observations made in oppo.site parts 
of the earth’s surface. 
Mr. JliniT A JCkmcjits of the Transit of Venus in 1874. — Some surprise was 
cxicasioned by tlie circumstance tliat ^I. I’uiseux liad deduced different re- 
sults than Mr. Hind from Loverrier’.s tables of the sun and Venus. INlr. 
Hind, having little faith in the efficacy of a re-exuinination of his own cal- 
culation.‘< by him.self, placed the matter in the hands of Mr. Plummer, the 
ai' distant at Mr. Bishop’s observatory, a very able and acute computer. The 
results of Mr. I’lmnmer's calculations accord so closely with those already 
