As has been impressed, however, the fact of actinic contrasts should be made very familiar to the photographic worker through measure ments and comparisons. The sun at 32,000,000 actinos against a sky of 512 actinos presents two adjacent surfaces with a contrast of 1 :64M, the sun being intrinsically about 64,000 times as intense as the sky. A newly painted white building in direct sunlight is about twice as intense as the sky. A head placed about the width of a window in from one of its casings toward the center of an ordinary room will be only from .L to 6< as intense on the shaded as on the lighted side. When a reflector is used so as to balance this contrast the darker side will have from ; to i the intensity of the lighter. This degree of contrast introduces the half tones of the picture and the detail in the shadows of the hair and clothing. Out in the open the sunny side of an object of uniform color throughout, will be only about 4 times as actinic as the shady side.
Any two actinicities may be compared on measuring that of each by any of the methods given in the problems. But when contrast alone is desired and not actual values, it may be found, provided the surfaces be of the same color, by tinting any sensitive product in the light near the brighter surface, to any convenient degree, and then observing the number of times longer required in the position of the weaker surface to secure the same tint. For example in taking the contrast between the sunny and shady side of some object out in the open, using the contact opening in the back cover of the meter book (p. 35) and a strip of p.o.p., expose to the sun and sky for one second then change the position of the strip of paper and on the shady side of the body expose to the sky alone long enough to bring the tint to the same depth. Four seconds will probably be found sufficient. Sunlighted subjects are not, as is usually supposed, exces sively contrasted, especially in the open. The lighted and less lighted, or as usually termed the lighted and shaded areas are however divided by very sharp lines and the impression of great contrast in such subjects is generally derived from the appearance of negatives made from them which have been under-exposed and over-developed.
From the standpoint of the photographic artisan each actinic value throughout the different planes of a subject is as "a tool in the hand," with which he must with absolute foreknowledge, create the tone scale from high-light to deepest shadow in his resulting picture. This he must do by first conceiving what result or tone scheme he desires, then by seeing that the actinic grada tions agree with this scheme after which the tool, which is the actual exposure, must be applied, so as to obtain the effect desired, as understood from the definition of the term "cor rect exposure," given on page 14. Actinicity
must therefore be studied as a fundamental ele ment in the photographic process just as are now considered in a quantitative sense the facts of diaphragm opening, speed of emulsion and exposure time, and these latter are in fact less fundamental than actinism, since they cannot be properly comprehended nor applied except in their relation to the actinicity and contrast of the subject photographed.
The actinic contrast of nature, as it presents itself ordinarily, is frequently beyond the capac ity or latitude of any emulsion to register. In view of the writer is a white cloud having an intensity of at least one thousand actinos. The inside casing of the window through which that cloud is observed measures but one actino. (In the average room it would measure only from a to an actino.) It has been found that no emulsion has so great a latitude as this. But few if any emulsions can normally register contrasts greater than 1 to 64 (p. 118).
In the case of the cloud and the window casing mentioned, if an emulsion having a latitude of 1 to 64 be used and the cloud be made to overcome the inertia of the emulsion 64 times, an intensity in the same view which would overcome the inertia only once must be at least 4 as actinic as the cloud, or must have 16 actinos of intensity and the window casing with its one actino could only do part of the work necessary to overcome the inertia once. On the other hand should the exposure be increased 16 times so as to give time for the casing to overcome the inertia once, then the image of the cloud would produce 16 times the exposure that the emulsion normally should endure. The sky by the side of the cloud would also have had an exposure equal to about 8 times the normal endurance of the emulsion and the cloud and sky would be no longer distinguishable from each other in the opacity planes of the negative. Furthermore the long duration on the emulsion, of the images of the cloud and the sky, 16 or 8 times as long as the endurance of the emulsion, would constantly illuminate the whole interior of the bellows and furnish suffi cient light reflected from its sides to overcome the inertia evenly over the whole emulsion. This uniform exposure would develop as "fog" in the negative. It is extremely important therefore that the worker recognize the fact that too great contrasts are to be avoided since they are beyond the capacity of emulsions to register.