American cinematographer (Jan-Dec 1926)

December, 1926 AMERICAN CINE M A T O G R A P H E R Eleven A Professional's Notes for Amateurs Part II Fundamentals and Principles By Jos. A. Dubray, of Lens Construction Given A.S.C. Lucid Explanation Although light travels with an extreme velocity, human ingenuity has accomplished the seemingly impossible feat of measuring this velocity to a great degree of accuracy. It would be inconsistent with the scope of these articles to enter into a detailed description of the means by which such measurements have been secured. Suffice to say that: Romer measured the velocity of light through astronomical observation of the respective positions of the earth and the first satellite of the planet Jupiter at six months' intervals. Foucault used a very ingenuous laboratory apparatus less than 14 feet long, through which a small beam of sunlight was submitted to a series of total and partial reflections and rotary deviation, which enabled him to derive a formula giving the velocity of light at a figure somewhat less but astoundingly near the results obtained by astronomical observation. Fizeau obtained his results through terrestrial observation between two points situated at nearly 30,000 feet from each other. A method similar to Fizeau's has been recently used in the mountains of California by Professor A. Michelson of the University of Chicago. The following table gives some of the results obtained:" Velocity per St cond in Vacuo. by In the Year Miles Kilometers ROMER 1675 190.000 304,000 FIZEA1 1844 196,000 SI 1,600 FOUCAULT 1850 1 S ; . 1 5 7 296.257 MICHELSON 1880 187,443 209.908 YOUNG & FORBES 1 SS2 188,314 101.382 NEWCOMB ISS2 187,413 299,861) CORM 1900 I87.7S0 S00 401) MICHELSON 1926 186,300 298,080 These are not all the complete data available on the results obtained in the solution of this captivating problem. We have chosen these, taking in consideration the precedence of effort, and the name of the scientist. These results obtained by different means, in a lapse of time covering a period of 251 years, have a truly amazing significance if we consider the slight differences in the figures obtained in such a delicate undertaking. WE shall now consider the behavior of a ray of light, incident to the smooth and highly polished surface of an opaque body; that is to say, a ray of light emanated by a luminous body, and falling upon such a surface. A very small portion of this light is absorbed immediately at the surface of the body or at a small distance from it, and is extinguished. Another portion of the incident light is scattered, or irregularly reflected in all directions. This scattered light is the light that renders visible objects that are not lumnious. If this phenomena did not occur, our world would be a strange one, engulfed in total darkness, broken by intense patches of light, emanated by the lumnious bodies or regularly reflected under the formal condition which we will presently expound. It is the scattering of the light that strikes the upper layers of the earth's atmosphere before sunrise and after sunset, that produces the well known phenomena of twilight. Remains that portion of light, which is regularly reflected, according to well established laws. We may note here the difference between the expressions of theory and laws. A theory is an accepted, but not thoroughly proven statement. In other words, a theory is the best explanation found by man of the causes producing certain phenomena. The supposed existence of the ether and of Huyghens light-waves* upon which the undulating theory of light is based, are not absolutely proven, but only serve to explain most of the phenomena produced by light. They leave a field open for investigation and supposition from which new theories arise, such as the electro-magnetic theory of light announced by Clark and Maxwell, and the electron theory of Lorentz. In opposition to a theory, is a law, which is an immutable proven fact, accepted beyond discussion, because in all cases in which the laic has been applied to practical uses it has completely agreed with the facts derived from observation. (Continue! on Page 23)