332 Mr. A. Claudet's Descrijrtion of the Photographometer. 



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. 



The openings of the moveable plate are parallel to each 

 other, and in the direction of the inclined plane. They are 

 seven in number. The first is 1 millimetre; the second, 2 

 millim. ; the third, 4 millim. ; the fourth, 8 millim.; the fifth, 

 16 millim.; the sixth, 32 millim. ; and the seventh, 64 millim. 

 Each opening is then the half of the following, and double of 

 the preceding one. After the operation we have seven (or 

 less, according to the intensity of light) separate images, the 

 different intensities of which represent the action of the light 

 during intervals of time in the geometric progression of:: 1 : 

 2:4:8: 16 : 32 : 64. These numbers are quite sufficient for 

 general observation ; we can multiply the effects by allowing 

 the plate to fall twice, thrice, or a greater number of times. I 

 adopted this progression because I found an arithmetic pro- 

 gression did not give an appreciable difference in the intensity 

 of the photogenic effect, a doubling of the action being required 

 to produce a marked difference of effect. If we wish to com- 

 pare the photogenic intensity of the sun's rays with those of 

 the moon, we allow the moveable plate to fall a number of 

 times sufficient for the moon's rays to produce an action, which 

 may be rendered apparent by the mercury. A few trials suf- 

 fice to obtain an effect from which may be calculated the pro- 

 portion of the intensities. 



The moveable plate containing the parallel proportionate 

 openings is furnished with a flap which opens and shuts freely, 

 and by which the openings may be protected from light when 

 the plate is to be raised again in order to fall a second time. 



A simple apparatus thus constructed will be a photographo- 

 meter certain for every preparation. It will be an index and 

 an infallible guide for the operator. It will indicate the in- 

 tensity of the photogenic light, and also the sensitiveness of 

 the preparation. But it becomes still more useful when con- 

 structed double; that is to say, when the moveable plate is 

 furnished with two equal series of proportional openings placed 

 in a line perpendicular to the direction of the inclined plane, 

 and the fixed plate pierced with two series of holes correspond- 

 ing to the openings in the moveable plate. 



By placing beneath each series of holes a different sensitive 

 surface, each of these surfaces will, during the fall of the 

 moveable plate, receive the same proportion of the same light, 

 and thus their different degrees of sensitiveness may be com- 

 pared. In this manner we learn the comparative sensitiveness 

 of different preparations of the iodide, of the bromo-iodide and 



