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observed by Sir John Herschel and myself; but it will be sufficient forour present purpose to state the general features of the impression.Some distance below the visible red ray, the paper will be found quiteuncoloured; on the part where the red ray fell, a tinting of red or pinkwill bo evident. The orange and yellow rays leave no stain, and thegreen in general but a faint one. In the place occupied by the blueray, the first decided darkening is evident, which increases through theindigo and violet rays, and extends some distance beyond them. Thoshaded wood-engraving, figure 11, will serve to assist this description.
I shall have occasion again to recur to this subject in my concludingsummary.
It will be evident from what I have stated, that tho maximum ofchemical action exists between tho bluo, indigo, and violet rays; and itmust bo plain upon consideration, that the refraugibility of these raysbeing different, they must also have different focal distances, or in plainerterms, that there are differences in tho distance from tho lens, at whicheach dissevered ray has its maximum of action. For photographicpurposes the indigo or violet rays are tho ones with which we workwith the best effect Sir John Ilorschel insists on the necessity of usingperfectly achromatic glasses, by which we unite all tho rays into onofocus. I have however found that tho several reflecting and refractingsurfaces of an achromatic lens retard the darkening action to some ex-tent, and by simply adjusting the paper to tho focus of tho violet rays,I have produced pictures more speedily with a common double convexlens, than when I have used an achromatic one, and as well defined inall their parts. This plan was first suggested by Mr. John Towson ofDovonport, boforo mentioned, and a paper on tho subject was publishedby that gentleman in the London and Edinburgh Philosophical Magazine.It has since appeared, that Professor Draper is in the habit of using thesame adjustment in taking portraits from tho life by the Daguerreotype.A few experiments will prove the advantage of using, what may bo calledfor want of better terms, tho chemical instead of tho luminous focus.Adjust tho camera, so that tho most porfect picture falls on tho paper;note tho time required to get a fair impression. Then haying placedequally sensitive paper in tho camera, bring it a little nearer tho lensthan before; under the same circumstances of light a better picture willbe produced in a shorter period: by repeating this, a point will bearrived at, where tho picture possesses greater sharpness of outlino, andaltogether exhibits a more decided effect. This is the focus required forphotographic purposes. It will be found, in general, at about one fortiethof tho whole focal distanco, nearer tho lens, than the true visible focus.
I have before mentioned, that on some papers, plates of glass have theproperty of deepening tho tints; we may avail ourselves of this in usingthe camera. Experiment will also show, that the photographic paperdarkens more readily when its sensitive coating is exposed to light stillwet, than when it is allowed to dry beforo exposure; therefore a greatimprovement is, to keep papers for camera purposes prepared with the