Mr. A. Claudet's Description of the Photographometer. 333 



chloro-iodide of silver, and of the various photogenic papers; 

 for it is indispensable, in making an exact comparison, to ope- 

 rate with the same light and during strictly the same space of 

 time, as it is known that the light varies from one minute to 

 another. 



When we only wish to make experiments of comparison 

 with different preparations, a much more simple and portable 

 apparatus will serve as well. In this the plate furnished with 

 parallel openings is immoveable, and fixed in the centre of a 

 frame having grooves underneath to keep it in contact with 

 another frame, in which we place the two sensitive surfaces 

 we wish to compare. Two plates, having round holes for the 

 passage of the light, cover the sensitive surfaces, which are 

 held by a shutter fastened at the back by a button. This part 

 of the apparatus resembles a Daguerreotype " passe partout" 

 with this difference, that in the place of the glass we have a 

 metallic plate furnished with holes. 



When the apparatus has been charged in a room shaded 

 from the daylight, it is carried out, and the frame containing 

 the sensitive surfaces is made to slide along by the hand, while 

 the frame containing the proportional openings is exposed to 

 the daylight or to the sun. With a little practice, a movement 

 ma}' be obtained regular enough to measure by this simple 

 apparatus, with sufficient exactness, the intensity of the light. 



By the double apparatus, whether it has the uniform motion 

 given by the inclined plane, or that less regular given by the 

 hand, we shall decide a very interesting question ; which is, to 

 know if each variation in the quantity and quality of light affects 

 equally the different photogenic preparations. We shall find, 

 perhaps, that certain lights require different preparations or 

 proportions to obtain the maximum of sensitiveness. 



In place of a single series of round holes upon each plate, 

 we can have two, three, four or five, which enable us to make 

 several experiments upon the same surface. On the paper, for 

 instance, we can prepare a zone for each series of holes with a 

 different substance ; and on the Daguerreotype plate we may 

 vary the coatings of iodine, bromine or chlorine, so that the 

 same surface will contain two, three, four, or five experiments 

 of comparison, serving to indicate the best method of prepa- 

 ration. 



The plate, fig. 4, shows the result of an experiment of this 

 kind, and is the exact representation of the plate itself after it 

 has received the mercurial vapour. The plate had been ex- 

 posed to the vapour of iodine in such a manner that one zone 

 had attained the first coating of yellow colour, a second zone 

 had reached the red, a third the blue-green, and the fourth, 

 after having passed through all these tints, had obtained the 



