UNIT ACTINOMETRY For practical use these conditions may be secured in the following manner. Procure a small pocket note-book with a tough, flexible paper cover and a small, shallow pasteboard box either square or circu lar and not over a half inch in depth, such as may be found in any drug store.
Bend down the top corner of the front cover as in dicated by the dotted line in figure 4 and press it down slightly so that it will hinge when lifted afterward. Cut a circular hole in the box cover mak ing its diameter the same as the thickness of the box thus establishing the f/1 form of opening illustrated by figure 3. Now lay the cover of the box on the corner of the book rather close to the hinge and with a pencil mark on the book cover the posi tion of the center of the opening. Cut at this position the contact opening described in the last paragraph. An irregular form is a help in observing the tinted spot to be made through it and the star shaped opening seen in figure 4 is made by first cutting a triangle and then notching each of its sides. As has been stated this should be only of a size sufficient to enable the spot tinted through it to be readily seen and examined, say approximately a quarter of an inch in diameter. Next cut a hole in the bottom of the box opposite the one in the lid and a little larger than the contact opening in the book cover when, after lining the box and cover with black velvet or painting it inside with some dull black medium and pasting or glueing the book cover, box and lid together so that all the openings will come opposite one another, the meter will resemble figure 5 and is ready for use.
It will be well also to make a contact opening in the back cover of the book, without a box attachment, for measuring the light at or inci dent upon surfaces as distinguished from that omitted by them. This distinction will pres ently be explained. Holding the book as though the back were the front, i.e., in an inverted
position, cut the contact opening at the top as the book may then be handled with the same convenience as though the opening were at the top of the front cover. Bend over the cover to hinge it as previously explained. Figure 4 illustrates perfectly this contact opening. The first leaf of the note book may be faced with a strong, unruled, white paper which will aid in distinguishing the tints to be obtained on the tinting strips.
These consist of sensitive strips of convenient length, a little wider than the contact opening, which may be cut from an ordinary roll of film or from one of the films from a film pack or better from some smooth, fast bromide (not gas light) paper. (See p. 44.) Similar strips of some gelatine printing out paper also should be pro vided. Small pockets of black paper may be pasted in the book to hold these strips or they may be carried safely between the leaves.
To use this f j 1 meter two basic conditions must be observed. One is that the distance from the surface whose actinicity is to be meas ured to the surface of the tinting strip, through the meter, must be somewhat less than the width of the surface being measured. Or in other words the meter must be held, in practice, a little closer to the area measured than the least width of that area, as illustrated by the accompanying figure. It is seen that the surface a does not fill the full angle of the cone of light which enters the meter and that surface a2 does so and hence complies with the conditions mentioned and which must be observed in using the meter. The other condition is that any area to be measured must be of quite uniform intensity throughout. Otherwise it is evident that a single intensity is not being measured but only an average measurement of different intensities.