This chapter is from the The History and Practice of the Art of Photography, by Henry H. Snelling. Published BY G. P. Putnam, 155 Broadway, 1849.
The last number (for March, 1849) of the "London Art-Journal, gives the following description of a recent improvement in Photographic Manipulation, and as I am desirous of furnishing everything new in the art, I stop the press to add it, entire, to my work.
"Since the photographic power of the solar rays bears no direct relation to their luminous influence, it becomes a question of considerable importance to those who practice the beautiful art of photography, to have the means of readily measuring the ever changing activity of this force. Several plans more or less successful, have been devised by Sir John Herschel, Messrs. Jordan, Shaw and Hunt. The instrument, however, which is now brought forward by Mr. Claudet, who is well known as one of our most successful Daguerreotypists, appears admirably suited to all those purposes which the practical man requires. The great difficulty which continually annoys the photographic amateur and artist, is the determination of the sensibility of each tablet employed, relatively to the amount of radiation, luminous and chemical, with which he is working. With the photographometer of Mr. Claudet this is easily ascertained. The following woodcuts and concise description will sufficiently indicate this useful and simple apparatus. [hipho_30.gif]
"For an instrument of this kind it is important in the first place to have a motion always uniform, without complicated or expensive mechanism. This is obtained by means founded upon the principle of the fall of bodies sliding down an inclined plane. The sensitive surface is exposed to the light by the rapid and uniform passage of a metal plate, A, B, (Fig. 31,) having openings of different length, which follow a geometric progression. It is evident that the exposure to light will be the same for each experiment, because the plate furnished with the proportional openings falls always with the same rapidity, the height of the fall being constant, and the angle of the inclined plane the same. Each opening of this moveable plate allows the light to pass during the same space of time, and the effect upon the sensitive surface indicates exactly the intensity of the chemical rays. The rapidity of the fall may be augmented or diminished by altering the inclination of the plane by means of a graduated arc, C, D, (Fig. 30,) furnished with a screw, E, by which it may be fixed at any angle. The same result may be obtained by modifying the height of the fall or the weight of the moveable plate. The photogenic surface, whether [hipho_31.gif] it be the Daguerreotype plate, the Talbotype paper, or any other preparation sensitive to light, is placed near the bottom of the inclined plane, F. It is covered by a thin plate of metal, pierced with circular holes, which correspond to the openings of the moveable plate at the moment of the passage of the latter, during which the sensitive surface receives the light wherever the circular holes leave it exposed.
"The part of the apparatus which contains the sensitive surface is an independent frame, and it slides from a dark box into an opening on the side of the inclined plane.
"A covering of black cloth impermeable to light is, attached to the sides of the moveable plate, enveloping the whole inclined plane, rolling freely over two rollers, R, R, placed the one at the upper and the other at the lower part of the inclined plane. This cloth prevents the light striking the sensitive surface before and after the passage of the moveable plate.
"It will be seen that this apparatus enables the experimentalist to ascertain with great precision the exact length of time which is required to produce a given amount of actinic change upon any sensitive photographic surface, whether on metal or paper. Although at present some calculation is necessary to determine the difference between the time which is necessary for exposure in direct radiation, and to the action of the secondary radiations of the camera obscura; this is, however, a very simple matter, and it appears to us exceedingly easy to adapt an instrument of this description to the camera itself.
"By this instrument Mr. Claudet has already determined many very important points. Among others, he has proved that on the most sensitive Daguerreotype plate an exposure of .0001 part of a second is sufficient to produce a decided effect.
"Regarding photography as an auxiliary aid to the artist of no mean value, we are pleased to record a description of an instrument which, without being complicated, promises to be exceedingly useful. In this opinion we are not singular; at a recent meeting of the Photographic Club, to which this instrument was exhibited, it was with much real satisfaction that we learned that several of our most eminent artists were now eager and most successful students in Photography. The beautiful productions of the more prominent members of this club excited the admiration of all, particularly the copies of architectural beauties, and small bits of landscape, by Messrs. Cundell and Owen. We think that now the artist sees the advantage he may derive from the aid of science, that both will gain by the union."
I hope the above description will induce our townsman, Mr. Roach, to successfully produce an instrument that will meet the wants of our artists in that part of the Daguerrean process referred to.