Mr. Cundell on the Photographic Camera Ohaira. 173 



ent decay diffused out with the first portions of oxygen. As, 

 therefore, more and more gas was evolved, the relative amount 

 of the nitrogen diminished. Thus the reason that the third tube 

 appeared to be richer in oxygen than the second, was owing 

 to its containing more gas. Any person, however, who is fa- 

 miliar with the physiological action of leaves will understand 

 these things without any further explanation. 

 June 15, 1844. 



XXVI. On a Combination of Lenses for the Photographic 



Camera Obscura. By Geo. S. Cundell, Esq^ 

 TTNTIL within the last few years, the camera obscura has 

 not been an instrument of sufficient importance to com- 

 mand much attention ; photography, however, has conferred 

 upon it a new character ; and many attempts have recently 

 been made to improve it, with various success. 



To adapt the instrument to photography, the principal ob- 

 ject is to obtain what is called " a,flat field," or a picture which 

 shall be in focus throughout, in the margin as well as in the 

 centre; but, along with a flat and focal field, it is necessary to 

 obtain a vivid and well-defined picture, with sufficient light to 

 act energetically ; and this last condition is especially required 

 when living figures are to form any part of the picture. 



It has been shown by Dr. Wollaston (and has been pointed 

 out in a late number of the Philosophical Magazinef), that a 

 lens of the meniscus figure, under certain conditions, will give 

 a picture which, although not absolutely flat, is much more so 

 than can be obtained by any other means; and, had he con- 

 templated the adaptation of the instrument to photography, he 

 would probably have made a small addition to it, similar to 

 that about to be described. 



All that his instrument requires, to make it a very perfect 

 one, is a higher intensity of light; and (without impairing its 

 other properties) it will be found, that that may be very effi- 

 ciently given, by the following arrangement : — 



Fig. 1. 

 A--..=.^_. 



gi.....---^--^ 



* Communicated by the Author. 

 t May 1844, S. 3, vol. xxiv, p. 321. 



