A Manual Of Photography

122 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. in the public mind to admit of the hope that any other may be adopted. If I might venture a suggestion, I would advocate a return to the term introduced by Niepce, whose processes we have considered—Heliography, Sun-drawing, which most clearly expresses the fact, leaving the question of the particular agent effecting the chemical change still open for examination. The annexed figure (16) shows the conditions as they are at present known. It was published many years since by me in a paper communicated to a periodical journal ; and since it has been confirmed by all my subsequent researches, it appears desirable to give it a more permanent record. From A to B exhibits the Newtonian spectrum, a and 6 being the rays which belonged to modern discovery previously to the recent observation that the peculiar blue rays seen in solution of quinine and some mineral oils belong to a yet higher order of refrangibility. The curves c, n, and E, represent the relative maxima of heat, light, and actinism, F being a second apparent maximum,—indicated in the red ray,—of the chemical powers. This may, however, be proved eventually to be a function of heat, since we know that calorific power will produce chemical change even when it is exercised as a radiant force. The operation of these antagonistic forces is somewhat re- markably shown over different regions of the earth. Advancing from our own lands towards the tropics, it is found that the difficulties of obtaining pictures by the solar influences increase; and, under the action of the glowing light of equatorial climes, a much longer period is required for impressing a photograph than is occupied in the process either in London or Paris. It has been stated by Dr. Draper, that in his progress from New York to the Southern States he found the space protected from chemical change by the yellow rays regularly increasing. The same result is apparent in the differences between the spring and summer. Usually in April and March photographs are more readily obtained than in June and July. In this present July (1852) there is a very general complaint that it is with exceeding difficulty that any photographs can be obtained: this is particularly felt to be the case with the collodion pro- cesses. It is very important to know that we may, by lowering the sensibility of the preparations, obviate these difficulties to a considerable extent. It is worthy of notice, that the morning sun, between the hours of eight and twelve, produces much better effects than can be obtained after the hour of noon : this was observed at a very early period by Daguerre. For drawings by application, this is but slightly, if at all, felt, but with the camera it is of