MEASURING THE LIGHT.
Now comes a very important part of the work—measuring the light— in order to give the correct exposure. The system of measurement which the author herewith presents has been practiced by him with the very best results. The system is based upon the fact that Solio paper made by the Eastman Kodak Company is constant in its sensitiveness to light.
To take the measurement of the light, expressible in seconds of time, proceed as follows : Provide a strip of thin cardboard or heavy opaque paper about two inches wide and six or eight long. Fold this across the center so the card board will resemble the cover of a small book and near the end of one flap cut a small square hole.
Place a fresh strip of Solio, with the face up, under the hole and cover the opening with a coin. Hold this cardboard as near as possible to where the face of the subject is to be placed and so that the hole exactly faces the light. Then remove the coin, expose 32 seconds and replace ; now raise the cover and look at the tint on the Solio. It will probably be very pro nounced. Pull the strip up slightly so as to bring it to a fresh place and give an exposure of i6 seconds. If the discoloration on the Solio is still very plainly visible, give another exposure of S seconds ; then one of 4 sec onds ; then one of 2 seconds in order that you may discover the shortest number of seconds that will give a just plainly discernible discoloration to the Solio. This is the " Solio Time." Therefore, the "Solio Mize" of anv light is the nienzber of seconds it has taken to give a barely discernible tint to the Solio paper in that light. On account of the great latitude of Eastman film, sufficient accuracy will be obtained by taking the Solio Time in even seconds such as t, 20 4, 8, 16, 32 seconds ; or 1, 2, 4, 8, minutes, etc.
It is not always prac tical to use a watch in counting seconds. After a
little practice, however, one can learn to count seconds with great accuracy without a watch by counting in the following way : (Begin with naught.) Naught one — half — and-- one one — half — and — two one — half — and — three, etc.
Practice counting by the second hand on a watch, giving the accent on the words, one, two, three, etc., so as to keep track of the number of seconds that have been counted. When the " Soho Time " at a certain point has been ascertained, it becomes very simple and certain to give the correct exposure by reference to the following tables : Table "A" is for use with the Kodaks having rapid rectilinear lenses and the numbered stop openings on the shutter.
Table " B " is for the fixed focus Kodaks which will not work with so large an opening as the others and which will therefore require a longer exposure.
It will be found that by carefully ascertaining the Solio Time and giving accurately the exposure indicated, perfectly timed negatives will always be obtained.
It is not difficult to give the exposure in just the number of seconds or fractions of seconds required by the table. Suppose the worker has a Kodak with a shutter having pneumatic attachment. Set the lever at " B " and to make I§ of a second exposure, press and release the bulb as quickly as possible. To give a quarter second, press and release the bulb while saying " quarter " at talking speed. For half a second say " Naught One-Half ", always pressing at the first and releasing at the last syllable ; and for one second, " Naught—One-half and One ". With the fixed focus Kodaks and Brownies, work the shutter lever of course instead of press ing the bulb. Use a tripod or other firm support.