Then, with pen and paper, proceed to divide the diameter of each stop into the focus, and state the result as a fraction of the focus, thus f/8.  For example, a Ross half plate rapid symmetrical has a focal length of 71/2 in.; for convenience reduce this to sixteenths=120.  A diaphragm measuring seven sixteenths will give the fraction f/17.  Now let us see if any of these stops correspond with Mr. Burton’s.  The first two in his table will only be found in portrait lenses, but we shall probably find one to correspond with the third, if we are using a doublet lens; with a single lens we won’t find any so large.  Having picked out those that correspond, and filled in the exposure for them, we have now to deal with the odd sizes.  Here is one, f/27, which is just half way between No. 16 and No. 32, but a moment’s thought will show that as the exposure increases as the square of the diameter, it won’t do to take the exposure half way between the two.

We have another factor to consider now:  that is, the rapidity of the plate.  If you use plates by a maker who has a name to sustain, you may be pretty confident that they are of fairly uniform rapidity, so after you have got into the way of working any particular brand, the best thing you can do is to stick to it.  The exposures in our table are for plates of medium rapidity in good spring light.  In my own experience I find that they just suit “thirty times” plates, or fifteen on the sensitometer; but then I like a full exposure with slow development, and I know that others find these exposures just right for “twenty times” plates developed in the usual way.  The most rapid plates in the market will not be overdone with half the given exposures.  It must always be borne in mind that an error of a fraction of a second in either direction may be corrected in development, and it is impossible to make a very serious error if you refer to the table.

We come now to the light.  If you depend on the eye entirely in judging the quality of the light, it will sometimes play you tricks.  The rays which are most active on the plates are those which have the least effect on the eye.  We can, however, by chemical means arrive at an exact estimate of the active power, and for this purpose an actinometer is used.  This is simply an arrangement whereby a piece of sensitized paper is exposed and allowed to darken to a standard tint, and by the time it takes to reach that tint the value of the light is judged.  Capt.  Abney has, however, pointed out that ordinary sensitized paper is not suitable for bromide plates, since there are conditions of light in which the plates will be fairly rapid while the paper will be very slow.  He gives a formula for a bromide paper, which is treated with tannin in order to absorb the bromine set free during exposure, otherwise the darkening would be very slight.  I used this paper for a while, but found it rather slow.  The tannin also turned brown on keeping for a week or so.  I then made some more, substituting for tannin potassium nitrite (not nitrate), which is colorless.  This was an improvement, but still it was just slow enough.