THERE is a new theory of light.

This in itself is not at all disturbing to the average person. Light has in no way changed and never will. But it is interesting to know that this mysterious and elusive thing which enables us to see and which has made photography possible, may be better understood and its action more clearly diagnosed because of photographic research.

The nature of light has been the subject of one of the most famous controversies of science. Sir Isaac Newton held that light consisted of distinct particles or corpuscles shot off from the source. Traveling with extreme velocity, they bombarded any object in their path and were reflected to the eyes of the observer, where they produced sight.

This theory had the advantage of explaining reflection very easily and with some difficulty Newton was able to explain the bending of a ray of light when it entered a transparent substance, such as glass or water.

But there were difficulties which displaced this theory for the "wave theory." According to this theory, light was held to consist of small waves of a definite high velocity traveling in a medium termed the "aether."

This theory was adopted at the beginning of the nineteenth century and has proved very satisfactory, especially after investigation showed that these waves could be treated as an electromagnetic disturbance in the aether.

Recently, however, a number of things have been observed which are difficult to explain by the wave theory, and it may be necessary to turn again to a theory similar to Newton's.

The origin of light is now ascribed, not to molecules or even the finest division of matter, the invisible atom, but to particles of negative electricity called "electrons," which are supposed to revolve around the nucleus of the atom which carries a positive electrical charge, the atom as a whole being electrically neutral.

We will admit this is a pretty deep theory for the layman, but the scientist insists that theory is nothing more than an explanation of facts, so we must take his word for it.

If a shock disturbs the revolving of these electrons, if they impinge on one another and are then attracted back to their nucleus, they give off pulses of energy in the form of waves in the aether whose frequency is proportional to the energy which the electron releases.

The wave length depends upon the frequency; the more waves in a given time the shorter they must be since the velocity of light is always the same. The wave length also determines the color of light. X-rays differ from light rays because their frequency is a thousand times as great as light. This gives them their penetrating power.

Portrait Film Negative, Artura Print By Sidney Riley Sydney, Australia.

Portrait Film Negative, Artura Print By Sidney Riley Sydney, Australia.

The new light that has been thrown on the question of the radiation of light is due to the study of photographic films. It has always been a mystery how a photographic emulsion adds up light during a long exposure, looking at it from the wave theory. The emulsion consists of microscopic crystalline grains, most of which have definite forms when viewed under a powerful microscope. Some are large, some are small but in clumps, and some are barely visible as specks under the highest powers of the microscope.

When these grains have received sufficient exposure to light, they become developable. But the difficulty has been to determine how they added up the light until they had enough, for it is quite certain that it is not necessary for them to get the light impression in any definite time.

For example, if a film is exposed under a telescope to the image of a star, an exposure of five minutes may not render any grains developable. Some action has occurred, however, for at the end of several five minute exposures a few grains can be developed, and after several hours of exposure, a good image of the star can be developed.

The hard thing to understand is how the sensitive grains can store up the billions of light waves that fail on them. We know they must because if the exposure is only half made and then started again weeks later, the grains will not have lost their record of the first exposure. The second exposure will start practically where the first left off.

It seems that the large grains are much more sensitive to light than the smaller ones, but if a number of grains even of the same size are sorted out under the microscope and exposed to a uniform flow of light, some will be developable before others, which would seem to indicate that even grains of the same size differ in sensitiveness.

If it were possible, however, to see these grains develop as fast as they received the necessary amount of light, you would hardly be able to imagine a continuous stream of light falling on them. You would imagine that the light was raining on the grains, hitting them here and there, and the effect would be that of raindrops falling on a dry sidewalk until finally the entire walk became wet.

Apply this theory to the effect of light on the grains of a sensitive film and you have the explanation of why large grains are most sensitive. They are more likely to be hit. This theory is made stronger because we know that light striking any portion of a grain makes the entire grain developable.

Portrait Film Negative, Artura Print By Sidney Riley Sydney, Australia.

Portrait Film Negative, Artura Print By Sidney Riley Sydney, Australia.

This assumption was made by Dr. Ludwik Silberstein, who is studying the problem in the Research Laboratory of the Eastman Kodak Company. He likens light to a rain of projectiles which he calls "light darts" and has been able to calculate the relation between the size and the number of grains that will be developable after a certain exposure, a relation which has been most accurately confirmed by special experiments in the same Laboratory. And from the rate at which grains of different sizes become developable, the average diameter (which appears to be very minute) of the projectiles or darts of light can be calculated. And this has been done.

On any chemical theory, it is very difficult to imagine that one grain is more sensitive than another, while a number of calculations tend to prove the new theory of a rain of light particles to be more likely correct than that of an unbroken stream of light.

Of course many questions regarding the new theory remain unanswered. Because it is new, it will have many difficulties to meet. It offers much food for scientific thought and will lead to many experiments.

If it is eventually accepted, it may prove of much value, as all scientific work eventually does, for all of the advances in the material advantages of our modern civilization may be traced either directly or indirectly to some laboratory of research in which a pioneer has discovered a new basic principle.