Spectroscope
2 of the 6 encyclopedias on this shelf carry an entry for Spectroscope. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
an instrument for observing spectra, or for spectrum analysis. With a single glass prism, the few most prominent lines in a solar spectrum may be seen by using a narrow slit to admit the light, which was the first great improvement made upon Newton's experiment, since a hole or wide slit gives confusion of effect. The second great improvement was to place a collimating lens behind the slit at its focal distance, whereby all the rays from the slit became a parallel bundle before passing through the prism. Finally a small telescope was mounted behind the prism to magnify and define the image thus obtained. The whole arranged on a table, with means of adjusting the collimating and eye tubes at the proper angles with the prism, forms the ordinary single-prism spectroscope. Further prisms may be added to increase the dispersion. Arrangements are often added for throwing the image of a micrometer scale upon the spectrum, or a reflecting prism may be placed over half of the slit to reflect the solar spectrum into the instrument for comparison with the tone spectra are compared with the solar lines, which are carefully mapped and form the standard of reference. By combining prisms of the different refractive and dispersive powers, a strong spectrum may be obtained without deflection. Such prisms may be contained in quite a small tube with slit and lens, and are called direct-vision spectroscopes, which are much used for microscopic observation. Instruments specially fitted for the purpose are called star spectroscopes, and there are also special sun spectroscopes, such being necessarily different in practical details from ordinary or chemical spectroscopes. It has become very usual to employ the spectra from diffraction gratings instead of prisms. The higher order spectra thus produced are very pure, and have the advantage of giving the lines in the true position due to their relative wave lengths alone, while prisms compress some groups of lines and extend others, according to the peculiar dispersion of the glass. Spectroscopes thus constructed are called grating spectroscopes. One of the largest spectroscopes in the world was completed in 1899 by Prof. John A. Brashear, an astronomer of Allegheny, Pa., for Dr. Hans Hauswaldt, scientist of Magdeburg, Germany. a This powerful concave grating instrument is 21 feet long and requires a room about 25 feet square in which to operate it. The grating used on the spectroscope has a six-inch aperture and is ruled with 110,000 lines. So accurately are these lines ruled that none of them varies more than 1998이이이 of an inch from the correct position; and so powerful is the instrument that whereas an ordinary spectroscope would show from 100 to 200 lines belonging to the spectrum of iron, this apparatus will reveal more than 2,000.
Aiton's Encyclopedia (1910)
. See Color
What Has Changed in the Last Century
Collier's in 1921 describes an instrument still recognisable to Newton: slit, collimating lens, prism or ruled grating, and a small telescope to view the result, with photography and eye as the only detectors. What was missing was any theory of why the lines appear where they do; quantum mechanics supplied that in the years just after, turning spectra from fingerprints to be matched against maps into direct readings of atomic energy levels. The eyepiece has since given way to photoelectric tubes and then to CCD and CMOS detectors feeding computers, and echelle designs pack the resolving power of Brashear's 21-foot instrument into a far smaller box. Spectrographs now do work no one on this shelf imagined: Hubble's 1929 measurement of galactic redshifts, the Doppler detection of a planet around 51 Pegasi in 1995, and laser spectrometers riding on Mars rovers.
Written for this edition, 2026.