Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

From: Noyes, William A. A Textbook of Chemistry. Henry Holt and Company, New York (1919) pp. 425-427.
Copyright © 1998 Richard A. Paselk

The simplest form of spectroscope is shown in Fig. 97. The prism A is of glass having a.high dispersive power. A narrow slit at B is illuminated by the flame or light which is to be examined. Between the slit and the prism is placed a.lens at C, which renders the rays of light from the slit parallel before they
reach the prism. The prism is set in such a manner that the angle of incidence on the first face is the same as the angle of emergence from the second face, as this gives the purest spectrum. The light is examined by means of the telescope D. A scale placed at E, illuminated by a light placed before it and whose image is reflected from the surface of the prism, serves to locate the position of the lines.
Such a flame as that of acetylene gives a continuous spec- trum, indicating that molecules of solid carbon in the white flame are vibrating in all possible periods required to give white light. If a Bunsen flame is placed before the slit and some com- pound of sodium, as sodium chloride, is introduced, the flame assumes a brilliant yellow color, and with a single prism spec- troscope the spectrum consists of a single, bright yellow line.
A spectroscope having several prisms, or a spectroscope using a metallic mirror (" grating ") ruled with many thousands of equidistant lines and which gives a diffraction spectrum, will separate the line into two lines situated close together. The wave lengths of the lines are 0.5896 and 0.5590 microns, the micron being the thousandth part of a millimeter. The physical significance of these lines seems to be that under the conditions of the flame either the sodium atoms as a whole or, more probably, portions of the sodium atoms or electrons within or around them vibrate at a definite rate, which is independent of the temperature. This rate is almost inconceivably rapid. The velocity of light is about 300,000 kilometers per second. This is equal to 3 X 10 14 microns, and since the wavelength of the sodium light is only 0.59 micron, the number of vibrations per second must be approximately per second.
There are two dark lines in the solar spectrum which coincide exactly with the bright yellow lines of the sodium spectrum. This is explained by supposing the interior of the sun to be an incandescent mass which gives out light vibrations of all wave lengths corresponding to the visible spectrum. The photosphere of the sun, on the other hand, consists of a gaseous envelope or atmosphere containing many different elements, among these sodium. The sodium atoms, if they have the power of producing light waves in the ether by their vibrations, must also be able to absorb waves of the same length from the ether, exactly as a tuning fork is set in vibration by sound waves of its own pitch, while waves of a different pitch do not affect it. The sodium atoms in the photosphere, therefore, absorb the waves of their own particular rate; and while they give the energy absorbed back again to the ether, they dissipate the energy by spreading it in all directions instead of allowing it to pass on toward the observer. The result is that the portion of the spectrum corresponding to the sodium vibrations will be rela- tively dark. By means of this principle it has been possible to show that more than thirty elements found on the earth are found also in the sun. One of these elements (helium), indeed, was discovered in the sun before it was found on the earth. For the purpose of comparing spectra it is convenient to place a right-angled prism before the slit of the spectroscope in such a manner as to cover one half of it. This may be made to reflect the light from a second flame into the slit in such a way that the spectrum from one flame will occupy the upper half of the field of vision while the spectrum from the other flame will occupy the lower half. In this manner the coincidence of lines in the two spectra may be readily observed.
In another form, known as the direct vision spectroscope, a series of prisms of different kinds of glass are so combined that one kind of glass counter-balances the mean refractive index of the other, while the dispersive effects are not counterbalanced. The effect is exactly the reverse of that in an achromatic lens. Such spectroscopes are especially suitable for the detection of the alkali and alkali-earth metals in qualitative analysis. By means of the spectroscope it is possible to detect 1/3000000 milligram of sodium. Only the methods used in studying radioactive sub- stances are more sensitive than this.
To obtain the spectra of iron, copper and other metals, which are volatile only at high temperatures, electric sparks from a Rumkhorf coil are passed between terminals of the metal, or, in some cases, between platinum wires, one of which is in a small cup containing a solution of a salt of the metal. The spectra of gases are observed in Plucker tubes, which have a narrow portion through which the electric discharge is passed.

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Last modified 22 July 2000