May 13, 1886] 
powerful lens the three points come out distinctly, and it 
is then an easy task to eliminate all that does not belong 
to the heavens. 
The construction of such a map, obtained by the 
apparatus as above described in three hours, would 
assuredly have demanded several months of assiduous 
labour by the ordinary processes. 
The following is the time of exposure requisite) to 
obtain the image of the stars.1 
Magnitude hatsms, * 6S: 
I © 0 0°005 
2 0 0 OI3 
3 0 0 003 
4 0 Oo 0°08 
5 = An eee cot a © 0 O72 
6 The extreme limit of magnitude atolra: 
of stars visible to the naked eye 5 
7 oe ee er ee a OO 13 
8 On Om 
9 O70 38 
10 eee poh dee = ae © 0 20 
II Mean magnitudeoftheasteroids 0 0 50 
12 rs a ae as o#)2) fo 
13 Oo) 5: 6 
14 Op1g 3.0 
15 The last of the stars visible with peas 
16 the aid ofthe great instruments 23 
All the above figures represent a minimum. To secure 
good reproductions on paper the time of exposure would 
hhave to be increased threefold. 
The above table shows that the time of exposure 
required in taking a star of the first, and that in taking a 
star of the last magnitude differ from each other as 
I: 1,000,000. (The relation adopted between the bright- 
messes of two consecutive magnitudes is 2°542.) 
Outside the construction of celestial maps, another field 
of study of great importance now created by photography 
may be cited, the discovery, namely, of the asteroids. 
The little stars fixing themselves on the plate as so many 
mathematical points, so to say, the planets are dis- 
tinguished therefrom, each by a little line perfectly defined 
indicating its proper movement in amount and direction 
during the time of exposure of the apparatus. It is in 
this way we have already succeeded in obtaining the 
trace of a small planet of the 11th magnitude which by a 
small line extremely well defined gave account of its 
march among the fixed stars. 
It will even be possible to study the movement of the 
satellites round their planet, and perhaps discover new 
ones. 
The study of the double and multiple stars will be 
greatly facilitated, and photography will be equally avail- 
able in the investigation of the parallaxes. 
Finally, photometry must be adduced as one of the 
branches of astronomy which will now be able to collect 
very valuable information through the utilisation of 
photography. 
In conclusion, it is worth while remarking how this 
fresh step in advance has sensibly enlarged the scope of 
man’s vision. In consequence of it we can now obtain 
the image of a star, which instruments of the same open- 
ing as those employed by photography would never of 
themselves have elicited out of their invisibility. 
PAUL ET PROSPER HENRY 
NOTES 
THE first soirée of the Royal Society this season took place 
last night. A large number of Fellows and visitors were present, 
and many objects of interest were exhibited. 
THE visitation of the Royal Observatory by the Board of 
Visitors is fixed this year for June 5. 
* For these results we have made use of the gelatino-bromide plates'of 
Monckhoven, 
NATURE 
37 
THE Royal Irish Academy is celebrating the centenary of its 
foundation this week. 
Dr. GILL, Her Majesty’s Astronomer at the Cape, has been 
elected Corresponding Member of the Imperial Academy of 
Sciences of St. Petersburg. 
THE fifty-seventh anniversary meeting of the Zoological 
Society was held on Thursday week. The chair was taken by 
Prof. Flower, LL.D., F.R.S., the President. The report of 
the Council on the proceedings of the Society during the year 
was read by Mr. P. L. Sclater, F.R.S., Secretary of the 
Society. It stated that the number of Fellows on December 31, 
1885, was 3193, showing a decrease of 62 as compared with the 
corresponding period in 1884. The total receipts for 1885 had 
amounted to 25,809/. 10s. 1d., being a decrease of 3129/7. as 
compared with the previous year. This decrease was mainly 
due to the falling off in the receipts under the head of admis- 
sions to Gardens, and in the amounts received for admission and 
composition fees from newly elected Fellows. The ordinary 
expenditure for 1885 had been 24,593/. 11s. 8d., against 
26,539/. 45. Id. for 1884. Besides that, an extraordinary ex- 
penditure of 491/. os. 6d. had been incurred, which brought up 
the total expenditure for the year to 25,0847, 12s. 2¢. The 
visitors to the Society’s Gardens during the year 1885 had been 
659,896, against 745,460 in 1884. The Davis Lectures on zoo- 
logical subjects, having been well attended during the past year, 
would be continued during the present season, beginning with a 
lecture on ‘‘ Pigs and their Allies,” by Prof. Flower, LL.D., 
F.R.S., on Thursday, June 3, at 5 p.m. The number of ani- 
mals in the Society’s collection on December 31 last was 2551, 
of which 756 were mammals, 1366 birds, and 429 reptiles. 
Among the additions made during the past year 21 were 
specially commented upon as of remarkable interest, and in 
most cases new to the Society’s collection. About 36 species 
of mammals, 15 of birds, and 4 of reptiles had bred in the 
Society's Gardens during the summer of 1885. The report 
concluded with a long list of the donors and their various dona- 
tions to the menagerie during the past year. 
WitH regard to the recent-explosion of the 43-ton gun, it is 
fortunate that it has happened without loss of life. Competent 
authorities, as seen from Col. A. Moncrieff’s letter (which we 
reproduce from the 7Zies) show that it could. How long 
are our gun factories to go on making guns condemned by 
easily-understood scientific principles? ‘* Col. Maitland’s in- 
teresting paper read at the Royal United Service Institu- 
tion on June 20, 1884,” Col. Moncrieff writes, ‘‘ published 
the process adopted at Woolwich in settling the types of 
the new steel breech-loading ordnance for the British ser- 
vice, as well as the proportions of the new guns on these 
types then in process of manufacture. Mr. W. Anderson’s in- 
vestigations, published in a lecture read before the Society of 
Arts on January 29, 1885, and also commented upon in the 
Engineer of February 6, 1885, clearly demonstrated that these 
guns were deficient in strength in front of the trunnions. It is 
a remarkable fact that several of the guns have now burst at the 
point and in the manner which could have been predicted by 
any one consulting Mr. Anderson’s demonstrated results. As 
the subject is of vital importance to the country, it would seem 
wise either to refute Mr. Anderson or accept his method and 
consult him ; his valuable service in having discovered the pre- 
yailing error and worked out this most difficult problem is too 
little known ; it would thus be utilised and acknowledged to the 
advantage of the service. By treating a gun as a heat-engine 
and accounting for every part of the energy generated by the 
explosion of the powder, he has, in a scientific and complete 
manner, proved that the metal crusher gauges from which the 
accepted curve of pressure is obtained are not to be relied on. 
