488 M. A. Claudet on the Dyiiactinometer, an Instrument 



far from that point. If the mouth or eyes are at the. mathe- 

 matical focus, the parts before and behind this plane wUl not 

 have the same degree of definition, but the effect will be tolerably 

 good for all. But if, without altering the focus of the object- 

 glass, we move the sitter forward or backward in such a manner 

 that the mouth and eyes ^nll be in the plane where the hands 

 were at first, or in the plane where the ears and arms were 

 before, we shall still have the face pretty well defined ; but in 

 the first case the hands, and in the second the ears and arms, 

 will have lost their correct forms. The nearer the apparatus is 

 to the object, the more conspicuous are these errors. When we 

 operate with veiy long focus object-glasses, the differences are 

 small, because the rays converge to the focus in angles less open. 

 With these object-glasses we can to a certain degree lose the 

 mathematical focus without any sensible difference, and for this 

 reason they are preferable wben the intensity of light or the 

 sensitiveness of the sui-face permits of their being used. 



From the preceding observations, it is obvious that the ope- 

 rator must always know the mathematical focus in order to direct 

 it on the most important part of the object, but not too far from 

 the extreme points. 



I have hesitated a long time to offer any hj'pothesis on the 

 cause of the variation between the two foci. I was in hopes that 

 some opticians, or persons more versed than I am in the mathe- 

 matics of the science of optics, would take up the subject, inves- 

 tigate it, and explain the cause of the phaenomenon. But this 

 subject being chiefly interesting to photographers, whose atten- 

 tion is constantly bent in obsei'ving facts of the most puzzling 

 kind continually interfering with the success of their difficult 

 operations, none but a practical photographer in the course of 

 his daily occupations can have the time and opportunity for in- 

 vestigating the question. 



Philosophers cannot form theories without facts ; and they are 

 generally little disposed to receive those upon wbich thsy have 

 not experimented themselves. In such a state of things science 

 is stopped in its progress, and no improvements are possible. 

 This is my excuse for venturing to start a theory, which 1 thraw 

 out more with a view to invite a useful and necessary contro- 

 versy, than to decide the point in a positive manner. The subject 

 belongs particularly to opticians, who have to construct lenses 

 for photographic purposes ; and it is for them to study the laws 

 of the new principle they have to deal with. Until now they have 

 employed all theii- abilities and skill in forming perfect instru- 

 ments free from sphencal and chromatic aberrations, as if they 

 had to construct telescopes. They have now another task to 

 perform : a good telescope might produce a very bad photo- 

 graphic camera, and a good camera a veiy bad telescope. 



