26o Cincinnati Society of Natural History. 



for instantaneous work all over the plate for which it is intended. 

 For time exposures the stop U. S. No. 32 should cover the same 

 plate with a definition, leaving nothing to be desired. Of course 

 exceptions must be made for very large lenses and difficult subjects. 



In focussing a given landscape on the ground glass every one 

 has observed that the center of the picture requires a shorter focal 

 length than the sides. This is due to the curvature of the field of 

 the lens; for that reason., when the center of a picture is in exact 

 focus, the sides will not be sharply defined, and vice versa. With- 

 out the use of a strong eye-piece there appears to be quite a margin 

 or space through which the lens may be moved back and forth 

 without affecting the definition. This space should be utilized in 

 favor of the sides after the center is well defined. 



The following is, I believe, a good method : Put in, first, a 

 stop one size larger than the one you know ought to give sharp 

 definition ; focus for the central line of distance with focussing 

 screw and for foreground with swing-back. By alternating once 

 or twice in that manner every thing will be sharp on this vertical 

 line. Then bring nearer together ground glass and lens until the 

 definition in the center is just beginning to lose in sharpness ; then 

 clamp the camera and put in a smaller stop if a rapid exposure is 

 not wanted. 



It may sometimes happen that the swing-back can not be used 

 for focussin'g purposes, as in the case of an architectural view, or 

 when in the central portion of the view are objects at different dis- 

 tances, requiring diff"erent focal lengths, such as a near bridge 

 under which can be seen a distant view. In either case a compro- 

 mise should be made, treating all principal parts of the picture 

 alike, and then using a very small stop, giving the required defi- 

 nition. 



In architectural work and groups greater sharpness is required 

 than for landscapes where those parts of the pictures only which 

 make it interesting need be absolutely sharp. 



When small negatives are made, with a view to enlargement 

 or for lantern slides, no pains should be spared to have them as 

 sharp as the lens and subject will permit. 



May 20, 1886. L. M. Petitdidier. 



