30

NEGATIVE PHOTOGRAPHS.

first washes only, applying tlio last sensitive wash, the moment beforeit is exposed to luminous influence. If, when wet, the paper is placedvery carefully on a piece of perfectly clear plate glass, the -wet side incontact with the glass, and then placed in the camera, the glass beingbetween tho paper and the lens, we avail ourselves of both the aboveadvantages at the same time. The paper must lie so close to the glassthat perfect reflection is everywhere obtained; tho minutest film of airbetween them admits of tho dispersion of the light, and gives a cloudi-ness to the photograph. Both Mr. Fox Talbot and Sir John Ilcrsclielare in the habit of adopting a similar manipulation.

It is worthy of notice that tho morning sun, between the hours ofeight and twelve, produces much better effects, than can bo obtainedafter tho hour of noon. For drawings by application, this is hut slightly,if at all felt, but with tho camera it is of some consequence to attend tothis fact. Wo are not yet in a position to record more than the fact, thocause of the difference is not yet detected; probably it may be found toexist in a greater absorptivo action of tho atmosphere, caused by tho ele-vation of aqueous vapour from the earth. But the experiments of M. Ma-laguti seem to imply the contrary, this philosopher having found that thechemical rays permeate water more readily than they do air; somo ex-periments of my own, are not, however, in accordance with M. Malaguti’sresults. In the neighbourhood of large towns it might be accounted for,by the circumstance of the air becoming, during tho day, more and moreimpregnated with coal smoke, &c., which offers very powerful inter-ruption to the free passage of chemical light. This will, however,scarcely account for the same being found to exist in tho open country,somo miles from any town. Until our meteorological observers arein the habit of registering the variations of light, by means of some welldevised instrument, we cannot expect to arrive at any very definiteresults. The subject involves some matters of the first importance inphotometry and moteorology, and it is to be desirod that our publicobservatories should be furnished with the required instruments.

C.—On Fixing the Negative PnoiOGRArns.

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The power of destroying the susceptibility of a photographic agent tothe further action of light, when the picture is completed by its influence,is absolutely necessary for tho perfection of tho art Various plans havebeen suggested for accomplishing this, which have been attended withvery different results; few if any of the materials used, producing theeffect, and, at the same time, leaving the picture unimpaired. Thohyposulphite of soda is decidedly superior to every other fixing material;but it will bo interesting to name a few others which may be used withadvantage.

The pictures formed on papers prepared with the nitrate of silver