Mr. A. Claudet's Description of the Photographometer. 331 



photographometer, because it indicates the combined results of 

 the photographic operation. 



My apparatus is very simple, and serves equally for pro- 

 cesses on paper or on metallic plates. It indicates the inten- 

 sity of the chemical rays at all moments of the day during 

 atmospheric variations, and at the instant we may wish to 

 operate. It serves also to compare the degree of sensitiveness 

 of the different photographic preparations. 



For an instrument of this kind (see Plate II. fig. 1), it is 

 important in the first place to have a motion always uniform, 

 without complicated or expensive mechanism. This I have 

 obtained by a 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 AB, fig. 2, having openings of different lengths 

 which follow a geometric progression. It is evident that the 

 exposure to light will be the same for each experiment, be- 

 cause 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 sur- 

 face indicates exactly the intensity of the chemical rays. The 

 rapidity of the fall may be augmented or diminished by alter- 

 ing the inclination of the plane by means of a graduated arc 

 CD, fig. 1, furnished with a screw E, by which it may be 

 fixed at any angle. The same result may be obtained by modi- 

 fying the height of the fall or the weight of the moveable plate. 

 The photogenic surface, whether it be the Daguerreotype 

 plate, theTalbotype paper, or any other preparation sensitive to 

 light, is placed near the bottom of the inclined plane F, figs. 1 &2. 

 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 sur- 

 face is an independent frame, and it slides from a dark box 

 into an opening on the side of the inclined plane. The fig. 3 

 represents the frame and the dark box, and by its inspection 

 the manner will be understood in which the sensitive surface 

 is placed in it before the experiment and carried to the mer- 

 cury-box after the operation. 



A covering of black cloth impermeable to light is attached 

 to the sides of the moveable plate enveloping the whole in- 

 clined plane, rolling freely over two rollers R, R' placed, the 



Z2 



