1866. ] — Dela Rue and Celestial Photography. 359 
and the hypotheses propounded by M. Berard, in 1812, which 
were reported upon by Berthollet, Chaptal, and Biot.* 
A few years passed away: Professor Schénbein discovered gun- 
cotton, and at the meeting of the British Association at South- 
ampton, in 1846, he introduced it as an important improvement 
upon gunpowder. As a destructive agent, gun-cotton has been 
slow in making its way as an agent for projecting cannon-balls 
or for rending rocks; but dissolved in ether, it forms that collodion 
which Mr. Archer, in 1851, taught us how to use in multiplying 
images of the beautiful, and the process to which it has given its 
name is now universally adopted, to the almost entire exclusion of 
every other kind of photographic manipulation. 
Ata very early period (1838-40), it was seen that the changes 
produced on the salts of silver by the sun’s rays might be used to 
render meteorological and other instruments self-registermg. In 
1838, Mr. T. B. Jordan, then secretary of the Royal Cornwall 
Polytechnic Society, devised and used photographic methods for 
registering barometers, thermometers, and magnetometers.t These 
methods, modified by Mr. Brooks and Mr. Ronalds, were subse- 
quently introduced into the observatories at Kew and at Green- 
wich, where, at the latter especially, under the direction of Pro- 
fessor Airy, a beam of artificial light now registers through each day 
and night every movement of those steel bars which tell us of the 
variations in the earth’s magnetic intensity, and of the occurrence 
of the strange phenomena known as “ Magnetic Storms,” now 
proved, by the investigations of General Sabine, to be intimately 
connected with those solar spots which are being explored—if the 
term is admissible—by Celestial Photography. While photo- 
graphy was making progress as an art, it was employed in a few 
hands as an aid in scientific investigations. Sir John Herschel 
especially used chemical compounds, sensitive to solar influences, to 
determine the relative values of the solar radiations proceeding from 
different parts of the sun’s disc, and this led to the determination of the 
* «To show clearly the great disproportion which exists in this respect between 
the energies of different rays, M. Berard concentrated, by means of a lens, all that 
part of the spectrum which extends from the green to the extreme violet, and he con- 
eentrated, by means of another lens, all that portion which extends from the green 
to the extremity of the red. ‘This last pencil formed a point so brilliant that the 
eyes were scarcely able to endure it, yet the muriate of silver remained exposed 
more than two hours to this brilliant point of light without undergoing any sensible 
alteration. On the other hand, when exposed to the otber pencil, which was much 
less bright and less hot, it was blackened in less than six minutes.’—‘ Report of 
the Commissioners ;’’ ‘Annales de Chemie.’ See also, “ Report on the Chemical 
Action of Solar Radiations:” ‘Transactions of British Association for 1850,’ 
vol, Ixxxy., p. 309. 
+ “On a New Method of Registering the Indications of Meteorological Instru- 
ments.” By T. B. Jordan. ‘Sixth Report of Royal Cornwall Polytechnic Society,’ 
1838, 
WOL,. 111. 2B 
