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Light Waves and Their Uses
of measurement. It is apparently something very indefinite. The visibility is not a quantity that can be measured, as we can a distance or an angle—unless, to be sure, we first define it. After defining it properly, we can produce, in accordance with that definition, interference fringes that shall have any desired visibility. By the use of fringes which have a known visibility we can educate the eye in estimating visibility, or we may have these standard fringes before us for comparison at the time of observation, and may then determine when the two systems are of the same clearness; and when they are of the same clearness, we say that the desired visibility is the same as that whose value is known from our formula. This is the more accurate method, and is the one which was finally adopted; but long before its adoption it was found that fairly accurate visibility curves could be obtained by merely agreeing to call the visibility 100 when it was perfect, 75 when good, and 50 when fair. Then 25 would be rather poor, 10 would be bad, and at zero the fringes would vanish. Of course, there would be a greater or less difference in what we should agree to call good, but in general we can tell where the fringes were half as clear as their perfect value, provided, of course, we had this perfect value given, etc.
As a matter of fact, however, it is not of the utmost importance to determine the visibility with great accuracy. We know that we can measure a minimum or a maximum independently of any scale, and these points are the really important ones. For example, a curve may come to zero gradually or abruptly — in both cases the distance between the two lines which produced the curve would be exactly the same. The two pairs might differ in character in other ways, but the distance between the two comjjonents of each pair would be the same. So, even without an absolute scale that we have tested, and even without any very great amount
Interference Methods in Spectroscopy
75
of experience in observation, we can get a very fair visibility curve, and from that a very fair conception of the nature of the spectrum of the particular source we are examining, by merely determining the points of maximum and minimum clearness.
Before discussing some of the visibility curves that have been obtained,
I should like to say a word concerning the source of light. When the source is under ordinary conditions, i. e[■ under atmospheric pressure, the molecules are not vibrating freely, and disturbing causes come in to make the oscillations not perfectly homogeneous.
Hence the light from such a source, instead of being a definite, sharp line, is a more or less diffuse band. In order to obtain the character of the line under the extreme conditions, i. e., under as small pressure as possible, the substance must be placed in a vacuum tube. The tube is then connected to an air pump and exhausted until the pressure in it is reduced to a few thousandths of an atmosphere.
When the exhaustion has become sufficient — the time depending on the particular degree of exhaustion required by the substance which we wish to examine — the tube is heated to drive off the remaining water vapor, sealed up, and is then ready for use. The residual gas is made luminous by the spark from an induction coil. In some cases the substance is sufficiently volatile to show the spectrum at ordinary temperatures; e. g., that of mercury appears after slight heating. In the case of such substances as cadmium and zinc the tube is placed in a brass box, as illustrated in Fig. GO, and heated until the substance is volatilized, a thermometer giving us an idea of the temperature reached.