S82 Mr. A. Claudet on the pt-incipal PJicenomena of 



I soon observed that anomaly, and imagined that it was 

 due to some errors in the respective position of the two 

 frames ; one holding the ground glass, and the other containing 

 the plate, which, by warping or some other causes, might 

 have been shifted to different distances from the object- 

 glass. 



Not being able to assign another reason for the error, I 

 constructed a camera obscura in which the ground glass and 

 the plate were exactly placed in the same frame. In doing 

 so I hoped to avoid the least error or deviation. But to my 

 surprise, the more I was correct in my adjustment, the less 

 I could obtain a well-defined Daguerreotype picture. This 

 proved lo me that I had to seek for another cause of the diffi- 

 culty ; and before going any further, I decided to try if the 

 usual focus did or did not really coincide with the photogenic 

 focus. For the experiment, I placed at a distance from the 

 camera obscura several screens on different planes : these 

 screens being covered with black lines, I could see them very 

 distinctly on the ground glass. I tried the focus on one of 

 the screens. To my surprise and delight, I found that inva- 

 riably the one which had come out well-defined on the ground 

 glass was confused on the Daguerreotype plate, and vice versa. 

 This was sufficient to prove to me the cause of the difficulty 

 I had been labouring under, viz. that the visual focus had not 

 coincided with the photogenic focus. But the most surprising 

 feature of that discovery was, that the photogenic focus was 

 longer than the visual focus. On first consideration it should 

 have been shorter, as the rays operating in photography are 

 the most refrangible. Although I could not at first under- 

 stand the cause of this anomaly, it was sufficient for me to 

 know that, in order to have a well-defined Daguerreotype 

 picture, I had only to set the focus on the ground glass for 

 an object nearer the camera at the distance indicated by the 

 experiment with the various screens. Continuing my experi- 

 ment, I found some lenses in which the photogenic focus was 

 shorter, and some others in which the two coincided. 



I communicated a paper on the subject to the Royal Society 

 and to the Academie des Sciences in May 1844, and from 

 that time photographers have been able to find the true pho- 

 togenic focus of their camera ; and opticians, who at first 

 denied the fact, have at last studied and considered the ques- 

 tion, trying to construct lenses in which the two foci should 

 agree. 



M. Lerebours of Paris was the first who, on my suggestion, 

 examined the subject; and he communicated a paper to the 

 Academie des Sciences, in which he explained the cause of 

 the difference. He stated that, by altering the proportion 



