484 jM. a. Claudet on the Di/nactinometer, an Instrument 



mine beforehand the comparative sensitiveness of the plates which 

 are to be used in the experiment with the dyuactinometer. By 

 this means I can try beforeliand several couples of plates, and 

 keep them as it were stamped with their degree of sensitiveness 

 untU I want to apply them to test the power of two lenses. The 

 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 canierje obscurje, eacli sup- 

 plied with the lenses tlie power of which I wish to compare, I 

 submit the two plates, each impressed with both the photogra- 

 phometer and dynactinometer, to the vapour of mercury, which 

 developes the two images on each plate. 



The number of spots given by the photographometer will in- 

 dicate the sensitiveness of the plate ; and in comparing the two 

 images given by the dynactinometer, accounting for tlie differ- 

 ence of sensitiveness of each plate, if there is arly, I am able at 

 once to determine the comparative power of the two lenses. 



During a great number of experiments, I have observed that 

 the power of the two lenses is not always iii the same ratio. It 

 appears that some kind of light affects one lens more than 

 another ; so that two different object-glasses, compared at 

 various times, do not always indicate the same proportion in 

 their power. 



A similar and as extraordinary an anomaly is observed in com- 

 paring the power of different parts of the same lens through 

 equal apertures. Generally, a given aperture on any extreme 

 zone of a lens has a greater photogenic power than the same 

 aperture on the centre or near it ; and the power of these two 

 different parts is not always in the same ratio, although each 

 will give a visual image of the same intensity. 



Before I had ascertained this fact, I had observed that the 

 use of diaphragms reducing the aperture, say to i, ^, or ■^, did 

 not always reduce the photogenic power in the same proportion. 

 Sometimes, if I was able to obtain an impression in ten seconds 

 with the whole aperture of the lens, I had no effect in twenty 

 seconds when using a diaphragm reducing the aperture exactly 

 one-half. 



Having mentioned this fact to several photographers, and 

 particularly to Mr. Malone, who in his photographic operations 

 is a most attentive observer, they have stated to me that they 

 had often met with the same anomaly in the Daguen-eotype as 

 well as in the Talbotype processes. I have been struck with the 

 analogy existing between these various facts and my experiments 

 on the variation in the distance separating the two foci, and 

 from them I am tempted to risk an hypothesis for the explana- 

 tion of the cause of that variation. 



