ADDRESS. XXXVIL 
were universally regarded as the finest that had yet been executed. Another 
process, in which gelatine is substituted for albumen, has been invented and 
successfully practised by M. Poitevin, a French officer of engineers; and by 
an ingenious method which has been minutely described in the weekly pro- 
ceedings of the Institute of France, M. Edmund Becquerel has succeeded 
in transferring to a Daguerreotype plate the prismatic spectrum, with all 
its brilliant colours, and also, though in an inferior degree, the colours of the 
landscape. These colours, however, are very fugacious; and, though no 
method of fixing them has yet been discovered, we cannot doubt that the 
difficulty will be surmounted, and that we shall yet see all the colours of the 
natural world transferred by their own rays to surfaces both of silver and 
paper. 
But the most important fact:in photography which I have now to mention 
is the singular acceleration of the process discovered by M. Niepce, which 
enables him to take the picture of a landscape illuminated by diffused light, 
in a single second, or at most in two seconds. This acceleration is produced 
by adding from 30 to 45 grains of honey to the white of each egg according 
to its size. By this process, he obtained a picture of the sun on albumen so 
instantaneously, as to confirm the remarkable discovery, previously made by 
M. Arago, by means of a silver plate, that the rays which proceed from the 
central parts of the sun’s disc have a higher photogenic action than those 
which issue from its margin. This interesting discovery of M. Arago is 
one of a series on photometry which that distinguished philosopher is now 
occupied in publishing. Threatened with a calamity which the civilized 
world will deplore—the loss of that sight which has detected so many brilliant 
phzenomena, and penetrated so deeply the mysteries of the material world 
—he is now completing, with the aid of other eyes than his own, those 
splendid researches which wili immortalise his own name and add to the 
scientific glory of his country. 
From these brief notices of the progress of science, I must now call your 
attention to two important objects with which the British Association has 
been occupied since its last meeting. It has been long known, both from 
theory and in practice, that the imperfect transparency of the earth's atmo- 
sphere, and the inequal refraction which arises from differences of temperature, 
combine to set a limit to the use of high magnifying powers in our telescopes. 
Hitherto, however, the application of such high powers was checked by the 
imperfections of the instruments themselves; and it is only since the con- 
struction of Lord Rosse’s telescope that astronomers have found that, in 
our damp and variable climate, it is but during a few days of the year that 
telescopes of such magnitude can give sufficiently distinct vision with the 
high magnifying powers which they are capable of bearing. Even in a 
cloudless sky, when the stars are sparkling in the firmament, the astronomer 
is baffled by influences which are invisible ; and while new planets and new 
satellites are being discovered by instruments comparatively small, the gis 
gantic Polyphemus lies slumbering in his cave, blinded by thermal currents, 
more irresistible than the firebrand of Ulysses. 
As the astronomer, however, cannot command a tempest to clear his at- 
mosphere, nor a thunder-storm to purify it, his only alternative is to remove 
Kis telescope to some southern climate, where no clouds disturb the serenity 
of the firmament, and no changes of temperature distract the emanations of 
the stars. A fact has been recently mentioned, which entitles us to antici- 
pate great results from such a measure. The Marquis of Ormonde is said 
to have seen from Mount Etna, with his naked eye, the satellites of Jupiter. 
