1864. | Photography. 155 
News’ have stated that, on certain days during particular hours, there 
seemed to be an almost total absence of actinic force. In some in- 
stances five and six times the ordinary exposure were given with very 
imperfect results ; and in other instances twenty times were tried with 
no effect. No particular atmospheric influence could be detected at work; 
and on subsequent days, apparently identical in light and clearness, 
photographic operations were conducted with their usual celerity. 
The cause of this great variation appears to have some connection 
with the dryness of the atmosphere, the days on which the absence of 
actinism was most marked having been intensely hot and free from 
humidity. It is much to be desired that a simple system of actino- 
metry should come into general use. The processes of Draper, Niépce 
de St. Victor, Bunsen and Roscoe, Herschel, Phipson, and others, are 
very useful, but rather too tedious for general use. What we want is 
some method of reading off the amount of actinism as simply as we 
read off the amount of heat with the thermometer. 
A most elaborate series of researches on the behaviour of chloride, 
bromide, and iodide of silver in the light, and on the theory of photo- 
graphy, has recently been published by M. H. Vogel.* The researches 
have extended over three years, and are of the most exhaustive cha- 
racter. We have only space to give some of the bare results which he 
has obtained, and must refer our readers for further particulars to the 
original memoir. The author considers that the action of light upon 
chloride and bromide of silver is first the production of a subchloride 
and subbromide, with liberation of chlorine and bromine, but that 
the iodide of silver undergoes no chemical change whatever. The 
action of acids and various saline solutions, especially nitrate of silver, 
has been studied very carefully, and some of the results are of con- 
siderable value. The effect of developing agents has been likewise 
examined, and the whole memoir constitutes one of the most im- 
portant contributions to the science of photography ever published. 
A valuable improvement has been inaugurated in the manufacture 
of lenses for photographic purposes. By the ordinary method of 
grinding and polishing, the surface is not left in a state of perfection 
anything approaching that required for astronomical glasses. For the 
usual photographic processes this surface is quite good enough, al- 
though, when carefully examined with a powerful glass, it will be seen 
covered with irregularities, the remains of the last stages of the grind- 
ing process. ‘T'o attain greater perfection entirely different means have 
to be employed, and the costly nature of this operation is one reason 
why telescopic lenses are so valuable. For some purposes, however, in 
which it is absolutely necessary to get perfect delineation, as in the 
copying maps, &c., a lens ground in the ordinary way would be inap- 
plicable, and perfection must not be hoped for unless the lens possesses 
a perfectly continuous spherical surface with the highest possible 
polish. Mr. Osborn, the photographer to the Melbourne Government, 
who is engaged in copying maps for the Melbourne Survey Office, has 
just ordered a lens from Mr. Dallmeyer, the cost of which is to be 
* Poggendorf’s ‘ Annalen,’ 1863, p. 497. 
