A Manual Of Photography

CHEMICAL ACTION OE THE SOLAB BAYS. 9 difficulty on prepared paper. Tins will probably be a useful application of the method: that it may be employed successfully, however, it is necessary that the paper be placed at but a small distance from the lens. (Davy.) “ In comparing the effects produced by light upon muriate of silver with those produced upon the nitrate, it seemed evident that the muriate was the most susceptible, and botli were more readily acted upon when moist than when dry—a fact long ago known. Even in the twilight, the colour of the moist muriate of silver, spread upon paper, slowly changed from white to faint violet; though, under similar circumstances, no immediate alteration was produced upon the nitrate. " Nothing but a method of preventing the unshaded parts of the delineations from being coloured by exposure to the day, is wanting to render this process as useful as it is elegant. An experiment on the darit rays of Hitter, by Dr. Young, included in his Bakerian Lecture,1 is a very important one. Dr. Young, after referring to the experiments of Kitter and W ol- laston, goes on to say: “In order to complete the comparison of their properties (the chemical rays) with those of visible light, I was desirous of examining the effect of their reflection from a thin plate of air capable of producing the well-known rings of colours. Eor this purpose I formed an image of the, rings, by means of the solar microscope, with the apparatus which I have described in the Journals of the Loyal Institution ; and I threw this image on paper dipped in a solution of nitrate of silver, placed at the distance of about nine inches from the microscope. 1n the course of an hour, portions of three dark rings were very distinctly visible, much smaller than the brightest rings of the coloured image, and coinciding very nearly, in their dimensions, witli the rings of violet light that appeared upon the interpo- sition of violet glass. I thought the dark rings were a little smaller than the violet rings, but the difference was not suf i- ciently great to be accurately ascertained: it might be as much as J- or As of the diameters, but not greater. It is the less surprising that the difference should be so small, as the dimen- sions of the coloured rings do not by any means vary at the violet end of the spectrum so rapidly as at the red end. The experiment in its present state is sufficient to complete the analogy of the invisible with the visible rays, and to show that they are equally liable to the general law, which is the principal subject of this paper :" that is, the interference of light. M B G. Sage, in the “ Journal de Physique, 1802, mentions a fact observed by him, that " the realgar which is sublimated at i philosophical Transactions, 1804.