oct.— dec. 1856.] Paper Process in Photography. 77 



pictures were developed in 23 and 24 minutes, J$ of the volume of 

 Gallic-acid, being added from the exciting solution. 



The amount of exposure for any focal length and aperture being 

 known — the time necessary for any other lens is found by the fol- 

 lowing rules, viz. : — 



With lenses of the same focal length, the time of exposure is 

 inversely as the square of the diameter of the aperture in the dia- 

 phragm. 



With the same aperture and different foci- — the time of exposure 

 is, directly as the square of the focal length. 



With different apertures and foci, the times of exposure are, in- 

 versely as the square of the diameter of the apertures, and directly 

 as the squares of the focal lengths. 



The correct time of exposure however is one of the Photogra- 

 pher's difficulties, and requires some considerable experience to ad- 

 just accurately under every kind of light. 



Developing Solution. 



Some hours before it is required to be used, fill a large stoppered 

 bottle with distilled water, put in a piece of camphor, the size of a 

 nutmeg, and some Gallic-acid, the exact quantity is of no conse- 

 quence, so that it be in excess. Upon an emergency the solution 

 of the Gallic-acid may be accelerated by putting the bottle (without 

 stopper) in a jug of hotwater. The Photographer however should 

 be careful to have a saturated solution, always, or there will be no 

 certainty in his practice. 



Filter as much of the above solution into the developing tray as 

 will cover it to the depth of at least one-eighth of an inch, and 

 then examine the state of your negative. 



If by the light of a taper, the outline of the upper part of the 

 buildings or trees,* is just plainly visible, the paper is in the best 

 state for developing and you should add to the Gallic-acid about 

 T V of its volume of the exciting solution, If the sky line is very 

 strongly marked, add ^ to and if the paper has been so long 

 exposed as to bring out the brighter parts of the view, you may 

 perhaps save it by omitting the Aceto-nitrate altogether or even, in 

 extreme cases, diluting the Gallic-acid, — adding a small quantity 



* " Technically known as the sky line." 



