D.—DIRECTIONS FOR TAKING PHOTOGRArilS.

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If four glasses, or coloured fluids be prepared, which admit respectivelythe blue, green, yellow and red rays, and we place them over an liydrio-dated paper, having an engraving superposed, it will be bleached under theinfluence of the blue light, and a perfect picture produced; while underthe rays transmitted by tho green glass, the drawing will be a negativeone, the paper having assumed, in tho parts which represent the lights,a very defined blackness. Tho yellow light, if pure, will produce thesamo effect, and the red light not only induces a liko change, but occa-sions the dark parts of tho engraving to be represented in strong lights:this last peculiarity is dependent on the heating rays, and opens a widefield for inquiry. My point now, however, is only to show that thedarkening of the finished photograph is occasioned by tho least re-frangible rays of light; whereas, its preparation is effected by tho mostrefrangible.

I know not of any other process which shows, in a way at once so de-cided and beautiful, the wonderful constitution of every beam of lightwhich reaches us; yet this is but one of numerous results of an analo-gous character, produced by theso opposite powers, both necessary tothe constitution of the solar beam, which is poured over tho earth, andeffects those various changes which give to it diversified beauty, andrenders it conducive to tho wellbeing of animated creatures.

Before quitting this branch of the art, it will be interesting to ex-amine the modifications which have been introduced by some conti-nental inquirers.

M. Lassaigne, who has claimed priority in the use of tho iodide ofpotassium, saturated his paper with a sub-chloride of silver, which wasallowed to assume a violet-brown colour, and it was then impregnatedwith tho iodidated solution.

M. Bayard simply allowed ordinary letter paper, prepared accordingto Mr. Talbot’s method, to blacken by light. He then steeped it forsome seconds in a solution of iodide of potassium, and laying it on aslate, he placed it in tho camera.

M. Ycrignon introduced a somewhat more complicated process. Hisdirections are,—White paper should first bo washed with water acidu-lated by hydrochloric (muriatic) acid, then, after being well dried,steeped in the following solution:—Water fourteen parts, with one partof a compound formed of two parts of muriate of ammonia, two parts ofbromide of sodium, and ono of chloride of strontium. The paper driedagain is passed into a very weak solution of nitrate of silver. There isthus formed, by double decomposition, a chloride and bromido of silver,which is made to turn black by exposing the paper to the light forabout lialf-an-hour. To use this paper, it is steeped in a very weak so-lution of the iodide of sodium, and placed, quite wet, into the cameraobscura, at the proper focus. In fine weather, M. Verignon states, theeffect is produced in twelve minutes. I have, however, never produceda good picture by this process in less than thirty minutes. A greatobjection to this mode of preparation is the very rapid deterioration of