The Element of Time

THE ELEMENT OF TIME This is one of the basic factors in the study of actinometry and in the practice of photography. As a child the writer believed the duration of a second to be inconceivably short and it is certain that children generally have a very abstract idea of its duration. A simple system for count ing seconds accurately might profitably be taught in the kindergarten by the use of a clock which ticks seconds. In this way the pupil would learn to think time rythmically, something as one feels the rythm of music.

In the daily practice of photography it is of fundamental importance that one should be able to count time correctly and the following method should be practised diligently: A quarter second " quer — ter." A half second " naught — one — half." One second " naught — one — half — and — one." Any number of seconds " naught — one — half — and — one, one — half — and — two, one — half — and — three," etc.

In learning this method it may be practised by placing a coin or other convenient object over any suitable spot or mark and uncovering and covering that spot as the different intervals of time are counted. For example to count and expose the mark for a half second, slip the coin off exactly as the word " naught" is spoken and return it on saying the closing word "half." This will be illustrated by the following arrange ments: (off) (on) (off) (on) " naught — one — half," " naught — one — half — and — one," (off) (on) " naught — one — half — and — one, — one — half — and — two," etc.

The importance of perfecting this method can not be overestimated. Workers may find di version in competing with each other in counting the longer intervals of time, say from a half minute to a minute or more. One should be able to count a minute with not more than two or three seconds of error.

Photographic shutters which mark off auto matically different intervals of time should be graduated to the following geometric intervals in seconds: 1, - A a., ,:8 5:2 x:24 , etc., or so far as the rapidity of the shutter will permit. Shutters as now made are usually marked with the following time intervals: 1, I, and It frequently occurs however that the intervals marked 1 and I second re spectively result in practically the same exposure and that about a half second. The time marked

on some shutters results in more than double that time. When the method of counting time above given is mastered, the errors of the longer intervals of such shutters as those of and 1 second, may easily be detected by the unaided ear.

Rather than use the automatic action of the shutter for the intervals of one half second and longer the index should be placed at "T," and such exposures made with two distinct pressures of the bulb. This is advisable as should the subject begin to move during an exposure the shutter may immediately be closed. Should the exposure be given with the automatic action this of course would be impossible. This pre mature closing of the shutter would result of course in some under-exposure but the negative might still be usable should the first movement of the subject not have been sufficient to blur the image. The quarter second exposure can best be given with the index at "B," and with a single pressure and release as in speaking the word "quar-ter," as instructed. The ideal shutter would begin the automatic exposures at i second leaving the half and one second and all longer exposures to be given by hand. In this manner the unreliable valve and plunger mechanism of shutters could be discarded en tirely as the quarter second and the shorter intervals no doubt could be given without it.

If in the solution of any problem in light the time interval be 512,000 seconds or greater, the number may be reduced to 8000 minutes or more, or if necessary to the equivalent time in hours, always considering however, 64 min utes as being freely interchangeable with one hour as 64 seconds are with one minute. The use of the geometric scale is thus preserved out measurably disturbing results in practice. Also the geometric interval of 1024 seconds or minutes and the greater intervals may be con sidered as even thousands and written 1M, 2M, etc. In reading these terms however the letter should not be pronounced but the word "thousand" instead, as, 1M for example is pronounced " one thousand," etc.