for measuring the Intensity of the Photogenic Rays. 483 



If a Daguerreotype plate receive the image of the dynactino- 

 meter during its revolution^ it is obvious that each segment in- 

 dicates an effect in proportion to the intensity of light and to 

 the time that it has remained uncovered ; also that the number 

 of seconds marked on the first segment visible is the measure of 

 the intensity of light at the moment of the experiment ; the 

 effect of each segment being in reality the degree of intensity 

 which can be obtained during the corresponding time. 



When we want to compare two object-glasses, they are adapted 

 to two camerse obscm'se placed before the dynactinometer. After 

 having set the focus of the two apparatus, they are charged each 

 with a Daguerreotype plate or a photogenic paper. When all is 

 ready, the flaps are opened at the moment that the dynactino- 

 meter commences its revolution, and they are shut when it is 

 completed. The plates are removed and the images brought out. 

 In comparing the result produced on each, it is easy to see which 

 object-glass is the most i-apid, and in what proportion. For 

 instance, if the arithmetical progression has been followed, 

 and on one of the plates or papers the number 4 of the great 

 circle is the fii-st visible, the conclusion is that it has been 

 necessaiy for the intensity of the light at that moment to operate 

 during four seconds in order to produce an effect in the camera 

 obscura ; and if, on the other plate or paper, the first seven seg- 

 ments have remained black, and the eighth segment is the 

 first upon which the light has operated, the conclusion vnll be 

 that the object-glass which has produced the effect on the first 

 plate or paper has double the photogenic power of the other. 



But if the geometrical progression has been followed, the same 

 expeiiment will show the image of the segment No. 3 repre- 

 sented on one plate, and that of the segment No. 4 on the other, 

 as having each the first degree of intensity ; and we have to draw 

 the same conclusion as regards the power of each object-glass. 



However, this conclusion would be exact only on the supposi- 

 tion that the two plates were endowed with the same degree of 

 sensitiveness ; for if they had not been prepared identically in 

 the same manner, we could not have the exact measure of the 

 comparative power of the two object-glasses. The difference 

 might be due, not to any difference in the power of the object- 

 glasses, but to the inequality in the sensitiveness of the two 

 plates ; although, in repeating the experiment several times, the 

 mean result might be sufficiently conclusive. But this difficulty 

 has not escaped me, and I have tried to avoid it.^ Being able, 

 by means of my photographometcr (see its description, Phil. 

 Mag. for Nov. 1848), to compare the sensitiveness of two plates 

 under the action of the same intensity of light and during the 

 same space of time, I avail myself of this instrument to deter- 



