. The book of photography; practical, theoretical and applied. this to the second third, and the sameto the la^^t third. For example, supposea negative is to be tested ; fill in paperand cover up two-thirds, as in Fig. assume any exposure to be correct,say six seconds. Give this exposure, thenturn off the light or shut off with shutter,and move the card, as shown in Fig. 329,giving half the assumed exposure, or threeseconds ; then shut off light once more. remove card, and give three seconds the first part will have had 6+3 + 3or twelve seconds, the second 3 + 3 or six,and t
. The book of photography; practical, theoretical and applied. this to the second third, and the sameto the la^^t third. For example, supposea negative is to be tested ; fill in paperand cover up two-thirds, as in Fig. assume any exposure to be correct,say six seconds. Give this exposure, thenturn off the light or shut off with shutter,and move the card, as shown in Fig. 329,giving half the assumed exposure, or threeseconds ; then shut off light once more. remove card, and give three seconds the first part will have had 6+3 + 3or twelve seconds, the second 3 + 3 or six,and the last three seconds. It frequentlyhappens that the frame will have to becovered with tissue paper to diffuse thelight. This must always be done whenthere is to be dodging or vignetting. Ithas been found that ordinary tissue paperonly allows one-third the amount of lightto pass therefrom. If the exposure fora plain negative is known, the exposurefor a vignette for same will, on accountof its being covered with tissue paper,therefore be three times as FlK .Shorn iNAl, KMii&UKJi. Devices for Timing Exposure. For short exposures the printer caneasily count, and a little practice enablesone to do this quite accurately. Thenovice should learn to count seconds,against a stop watch or seconds handwatch. Start counting with the hand onsixty, and without looking at the watchcount up to sixty again and stop thewatch. This will tell you at once whetheryou are contracting a habit of countingtoo slowly or quickly. Short exposures arebetter counted than timed by a watch, buta pendulum may be used. A metionome(Fig. 329) as used for beating time in musiccan be employed, or a very simple arrange-ment is shown in Fig. 330. It consists merelyof a weight a on a length of rod B connectedto a lead block c by a piece of clock spring,and as it swings causes the catch e tovibrate and sound the seconds. Counting-is a little tedious, and an electric alarm is M 0f^-.\-, i 10 w 9m4 m vT l^ 1
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