International photographer (Jan-Dec 1934)

fwenty-two T h INTERNATIONAL PHOTOGRAPHE R January, 1934 Origin of the Point Source Theory of Light Intensity By F. Morris Steadman EPLER, more than 300 years ago, when he glpl arranged his study plan for the schools, knew full well that things were made visible by the play of light from whole primary and secondary light sources and expanses, as from the sun, the sky, and from the surfaces of lighted objects about us. He knew that whole window openings illuminated rooms and that, at night, things were made visible by the functioning of whole flames. There was no need that he should have ignored these natural light conditions, as will be shown, and the fact that he did so in his teaching plan, is a mystery that needs explaining. The present practically total popular ignorance of light is seen in the almost universal practice of "snap shooting" amongst amateur photographers and the custom of "guessing" the exposure amongst professionals. Dr. Woodhull, of Columbia University, and many other educators, have revealed to me their dissatisfaction with the Point Source theory of light instruction because of its failure as explained above. We are to be privileged soon, I believe, to see this study of the technical point source dropped from the school books and the study of natural light sources substituted. This series of articles is to hasten somewhat this reform. Two different patterns of light play will now be described : The light from the whole hemisphere of sky converges upon each accessible grain of dust on the earth, but on a dark night a light source placed on the earth would spread its light outward through this same hemisphere of directions. One pattern is just as true to nature as the other. This is a matter of illumination patterns, not of intensity. For some unnecessary reason the scientists have visualized only the pattern of illumination which spreads from small sources. This is the unfortunate and even disastrous limitation from which Flammarion and Count Rumford freed themselves, as will be explained further on. We will now see how Kepler was trapped into ignoring the whole flame and using a technical point source instead. Imagine him at work with a small flame in a room. Half way to some opposite wall he suspends an opaque card, say a foot square. He sees a shadow on the wall that is approximately two feet square and which has about four square feet in it, or four times the area of the card. But he notes that this shadow measurement is not precise, because of a blend, which we call the penumbra, all around its edges. This blend on the sides of the shadow is equal in width to the width of the flame and at the top and bottom of the shadow it is equal to the height of the flame. Exactness must be secured. He lets the light shine through a small hole, say a fourth of an inch in diameter, the card with this hole being held against the flame. The penumbra is now reduced to the quarter inch blend. It is clear that to make this shadow perfectly sharp at the edges and make the shadow iust four times the area of the card, the source must be reduced to a technical point. We find then, that in order to reduce to an accurate geometrical design the truth that light at double distance spreads over four times the area that it does at distance one and do it in the single pattern of spreading light to which he was restricted, he had to visualize the light as spreading from a technical point, thus making it necessary to ignore the very cause of the brightness — the whole flame. Will the reader please try to visualize the situation in which the wise Kepler, in explaining an effect, annihilated the cause thereof? The present popular ignorance of light, after using the above erroneous scheme for 300 years, will continue just as long as the plan is retained in the books. Now let us see how a different visualization of light play might have avoided that error and led Kepler naturally into the formation of a plan that would have permitted the study of whole light sources, instead of a technical point source: Suppose that while out of doors he had appreciated the fact of the light converging upon him from the whole sky. The sky is a hemisphere above and around him and he a very small object resting in its light. With this pattern in mind he could have arranged a grain of white chalk on some dark surface and seen the light from his small frame converge upon it to illuminate it. On holding the grain very close to the flame he would have noticed the increased convergence of the light to it and its correspondingly greater intensity, while touching the grain to the flame he would have realized that the light was coming to it from a whole hemisphere of directions, just as the light came to him from the whole sky when out of doors. Had Kepler visualized this convergent pattern of light play instead of that of spreading light, we might never have heard the term "Point source of light" as associated with intensity. He would have discovered that the point seat of intensity, in the molecule, would have solved the whole sphere of natural luminosity, in his teaching plan, so that students could have studied nature's plan of light play as it actually functions about us. It is encouraging to note that Camille Flammarion, in explaining the sun's influence for light and heat on the different planets, ignored the Point Source theory and the pattern of spreading light entirely. In the book, "Astronomy for Amateurs," the English translation of one of his popular French books, on page 157, we find: "At that distance" (that of Jupiter from the sun) "the sun displays a diameter five (5.2) times smaller than that which we see and its surface is twenty-seven times less extensive; accordingly this planetary abode receives on an average twenty-seven times less light and heat than we receive." In other words : As the planets get closer to the sun, the sun gets larger in the sky, and their brightness increases in the same ratio. It must be understood that this is precisely the same law that functions on turning up a flame or raising a window shade, when the distance is fixed. As stated in the preface of my book, "Unit Photography," this truth of the importance of the convergent light pattern in creating intensity came to me independently, about the year 1895, as the result of my work with ordinary windows while making portraits in private homes. It is of course true that Flammarion and Count Rumford used the same truth much earlier. Both these [Turn to Page 29] Please mention The International Photographer when corresponding with advertisers.