A Manual Of Photography

162 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY, dynactinometer 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 rapid, and in what pro- portion. 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 first visible, the conclusion is that it has been necessary for the intensity of the light at that moment to operate during four seconds in order to produce an ebbet in the camera obscura; and if, on the other plate or paper, the first seven segments have remained black, and the eighth segment is the first upon which the light has operated, the conclusion will 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 experiment will show the image of the segment No. 3 represented 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 prepaied 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 the inventor, and he has tried to avoid it. Bein»" able, by means of the photographometer, to compare the sensitiveness ‘ of two plates under the action of the same in- tensity of light, and during the same space of time, he availed himself of this instrument to determine beforehand the, compara- tive sensitiveness of the plates which are to be used in the ex- periment with the dynactinometer. By this means we can try beforehand several couples of plates, and keep them as it were stamped with their degree of sensitiveness until we want to apply them to test the power of two lenses. Ihe impression is made on one-half of the plate, leaving the other half for the image of the dynactinometer. After having operated in the two cameræ obscurre, eaeli sup- plied witli the lenses the power of which we wish to compare, we submit the two plates, eaeli impressed witli both the photo-