The approaching completion of the 100-inch telescope of the Mount Wilson Observatory led me to suggest to Professor Michelson, before the United States entered the war, that the method be thoroughly tested under the favorable atmospheric conditions of Southern California. He was at that time at work on a special form of interferometer, designed to determine whether atmospheric disturbances could be disregarded in planning large-scale experiments. But the war intervened, and all of our efforts were concentrated for two years on the solution of war problems.[*] In 1919, as soon as the 100-inch telescope had been completed and tested, the work was resumed on Mount Wilson.
[Footnote *: Professor Michelson's most important contribution during the war period was a new and very efficient form of range-finder, adopted for use by the U. S. Navy.]
The principle of the method can be most readily seen by the aid of an experiment which any one can easily perform for himself with simple apparatus. Make a narrow slit, a few thousandths of an inch in width, in a sheet of black paper, and support it vertically before a brilliant source of light. Observe this from a distance of 40 or 50 feet with a small telescope magnifying about 30 diameters. The object-glass of the telescope should be covered with an opaque cap, pierced by two circular holes about one-eighth of an inch in diameter and half an inch apart. The holes should be on opposite sides of the centre of the object-glass and equidistant from it, and the line joining the holes should be horizontal. When this cap is removed the slit appears as a narrow vertical band with much fainter bands on both sides of it. With the cap in place, the central bright band appears to be ruled with narrow vertical lines or fringes produced by the "interference"[*] of the two pencils of light coming through different parts of the object-glass from the distant slit. Cover one of the holes, and the fringes instantly disappear. Their production requires the joint effect of the two light-pencils.
[Footnote *: For an explanation of the phenomena of interference, see any encyclopæedia or book on physics.]
Now suppose the two holes over the object-glass to be in movable plates, so that their distance apart can be varied. As they are gradually separated the narrow vertical fringes become less and less distinct, and finally vanish completely. Measure the distance between the holes and divide this by the wavelength of light, which we may call 1/50000 of an inch. The result is the angular width of the distant slit. Knowing the distance of the slit, we can at once calculate its linear width. If for the slit we substitute a minute circular hole, the method of measurement remains the same, but the angular diameter as calculated above must be multiplied by 1.22.[*]