OCT.— DEC. 1856.] Paper Process in Photography. 77 
pictures were developed in 23 and 24 minutes, 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 diffA*ent 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 l«ast 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 
of its volume of the exciting solution. If the sky line is very 
strongly marked, add J-g- 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." 
