CHEMICAL ACTION OF THE SOLAR RAYS. 9
difficulty on prepared paper. This will probably be a usefulapplication of the method: that it may be employed successfully,however, it is necessary that the paper be placed at but a smalldistance from the lens.(Davy.)
«In comparing the effects produced by light upon muriate ofsilver with those produced upon the nitrate, it seemed evidentthat the muriate was the most susceptible, and both were morereadily acted upon when moist than when dry—a fact long agoknown. Even in the twilight, the colour of the moist muriateof silver, spread upon paper, slowly changed from white to faintviolet; though, under similar circumstances, no immediatealteration was produced upon the nitrate.
« Nothing but a method of preventing the unshaded parts ofthe delineations from being coloured by exposure to the day, iswanting to render this process as useful as it is, elegant.”
An experiment on the dark rays of Ritter, by Dr. Young,included in his Bakerian Lecture, is a very important one. Dr.Young, after referring to the experiments of Ritter and Wol-laston, goes on to say:“In order to complete the comparison oftheir properties(the chemical rays) with those of visible light,T was desirous of examining the effect of their reflection from athin plate of air capable of producing the well-known rings ofcolours. For this purpose I formed an image of the rings, bymeans of the solar microscope, with the apparatus which 1 havedescribed in the Journals of the Royal Institution; and I threwthis image on paper dipped in a solution of nitrate of silver,placed at the distance of about nine inches from the microscope.Tn the course of an hour, portions of three dark rings were verydistinctly visible, much smaller than the brightest rings of thecoloured image, and coinciding very nearly, in their dimensions,with the rings of violet light that appeared upon the interpo-sition of violet glass. I thought the dark rings were a littlesmaller than the violet rings, but the difference was not suffi-ciently great to be accurately ascertained: it might be as muchas J or i of the diameters, but not greater. It is the lesssurprising that the difference should be so small, as the dimen-sions of the coloured rings do not by any means vary at theviolet end of the spectrum so rapidly as at the red end. Theexperiment in its present state is sufficient to complete theanalogy of the invisible with the visible rays, and to show thatthey are equally liable to the general law, which is the principalsubject of this paper:” that is, the interference of light.
M. B. G. Sage, in the« Journal de Physique, 1802,” mentionsa fact observed by him, that“the realgar which is sublimated at