NATURAL PHILOSOPHY. 143 



THE PHOTO GRAPHOMETER. 



AT a late meeting of the Paris Academy of Sciences, M. Clandet 

 communicated a. description of his newly invented instrument, for in- 

 dicating to the 'photographer the intensity of the chemical rays, and at 

 the same time the sensitiveness of his preparation. The apparatus is 

 very simple, and serves equally for processes on paper and on metallic 

 plates. It indicates the intensity of the chemical rays at all times 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. It is necessary that an in- 

 strument of this kind should have a uniform motion without intricate 

 machinery, and this is obtained by a means founded upon the princi- 

 ple 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, having openings of different lengths which follow a geometric 

 progression. It is evident that the exposure to the light will be the 

 same for each experiment, because the plate falls always with the 

 same rapidity, the height of the fall being constant, and the angle of 

 the plane always the same. The photogenic surface, whether it be 

 the Daguerreotype plate, the Talbotype paper, or any other sensi- 

 tive preparation, is placed near the bottom of the inclined plane, 

 and is covered with a thin plate of metal pierced with circular holes, 

 which correspond to the openings of the moveable plate. By placing 

 beneath each series of holes a different sensitive surface, each of these 

 will receive the same proportion of the same light, and thus the dif- 

 ferent degrees of sensitiveness may be compared. It is indispensable, 

 in making an exact comparison, to operate with the same light and 

 during the same space of time, as it is known that the light varies 

 from one minute to another ; this is accomplished by the photograph- 

 ometer. M. Claudet announces that this apparatus has furnished him 

 with a very extraordinary fact, which, however, he does not give as 

 precisely correct ; but he thinks that he cannot be far from the truth in 

 stating, that the pure light of the sun modifies the bromo-iodized sil- 

 ver plate, communicating to it an affinity for mercurial vapor, which 

 produces the white image in the Daguerreotype, in about the thou- 

 sandth part of a second. He made the experiment by admitting the 

 light of the sun through an opening of a French millimetre in size, 

 and this opening passed over a space of 350 millimetres in a quarter 

 of a second, so that the light could not have acted on the plate more 

 than the thousandth part of a second. It is suggested that this in- 

 strument may be used to ascertain the effect of the compound light, 

 and that of the different separated rays of the solar spectrum ; how 

 much photogenic light is lost by reflection from parallel mirrors, prisms, 

 and other substances, and by refraction through lenses ; the propor- 

 tion of photogenic rays in the light obtained from various sources, in- 

 cluding that produced by electricity ; if the photogenic light varies 

 with the height of the atmosphere and with the changes of tempera- 

 ture, and if it is affected by the electrical state of the atmosphere ; 

 and, lastly, what is the proportion of the photogenic rays at each hour 

 of the day, and at different points in space at a given moment 



