§ 2. The Spectroscope.

183. Its Essential Parts.-The spectroscope is an instrument for observing the spectrum. Fig. 100 shows its simplest form, and the relation of its parts. I represents the light, which may be from any source, natural or artificial, the spectrum of which is to be examined. A is a tube, closed at S, but in the end of which there is a vertical

slit, opened and adjusted by a slide or screw (205). This slit is a very important part of the instrument, and is formed by knife-edges of the most unchangeable material, finished with great accuracy, so as to give a perfect line, though not more than of an inch in thickness. The light entering the slit, passes through a tube called the collimator, containing a lens, by which the rays are made parallel before falling upon the prism. The rays emerge

from the opposite face of the prism refracted, yet only slightly dispersed, so that the spectrum S is but little larger than the width of the slit. In order to observe it of a sufficient size, at a short distance, a magnifyingglass or small telescope, F, is employed. The collimator, the prism, and the spy-glass, are therefore the essential parts; and in use the prism requires to be covered to exclude the interfering light.

184. Measuring the Spectrum.-But for scientific purposes the instrument requires the most accurate means of measuring the spaces of the spectrum. For this purpose a third tube has been added, as shown in Fig. 101 at S. At its outer end there is a glass plate, m, upon which is en

graved or photographed a scale of minute divisions.

A

lamp, K, throws the image of this scale through the tube and lens, so that it falls upon the face of the prism at n, at such an angle as to be reflected by the polished surface of glass through the telescope F to the eye. The scale is permanent, and parallel with it the observer sees the spectrum

FIG. 101.

m

Compound Spectroscope.

of whatever light is employed, and can thus fix and compare the position of the lines with exactness.

185. The Mounted Instrument.-The foregoing figures show a mere skeleton of the parts, for explanation: Fig. 102 represents the construction of the instrument as ready for use. A is the collimator-tube, the slit not being visible. A gas-burner is represented as the source of light; and a stand is shown beside it, with an arm for supporting in the flame any substances it is desired to experiment with. B is the telescope furnished with a guard to screen the eye from extraneous light. C is the tube with the scale for measurement, and a candle for projecting the image.

186. Direct-Vision Spectroscopes.-It would obviously be an advantage if the slit, lens, prism, and telescope, were

S

*

all in a straight line, so that the instrument could be applied directly to the light to be examined. This result is gained in the direct-vision spectroscope. If two exactly

similar prisms are combined in opposite positions, as in Fig. 103, AB, the changes impressed upon a ray by the first will be counteracted by the second. But, if the prisms differ in refractive angles, or density of material, this counteraction will not be complete. The deviation may be corrected, which will give a straight path for the light, but the dispersion may be but partially corrected, which will leave a spectrum. This depends upon the principle that the disper

sive power of various kinds of prisms is not exactly in the

Fig. 103.

Counteraction of Prisms.

proportion of their refractive power. Hence, if two crownglass prisms, PP (Fig. 104), are combined with a flint glass prism, P', of greater angle, and in a reversed position, the combination will give a spectrum in the line of sight. A train of prisms thus arranged, and combined with a spyglass,

forms the direct-vision spectroscope, which is shown mounted upon a stand, S, in Fig. 105. It may

be detached from the stand, unscrewed at the centre, and placed in a portable case. These

spectroscopes

are sometimes made

FIG. 104.

so small that they may be conveniently carried in the pocket.

§ 3. Spectral Lines.

187. Newton's Spectrum imperfect.-Newton used the light from a round hole in a window-shutter, so that his

FIG. 105.

I

Direct-Vision Spectroscope.

spectrum consisted of a series of overlapping images of the aperture, by which the colors were slightly mixed. In this way the deeper mysteries of the spectrum could not be disclosed; and for one hundred and twenty-seven years no progress was made in this branch of knowledge. But in 1802, Dr. Wollaston examined

the spectrum formed by a narrow opening, and found that, instead of being so pure as was always supposed, it was crossed in various places by dark lines. The discovery, however, although it was the initial step of modern spectrum analysis, excited no interest at the time.

188. Fraunhofer's Lines.-The dark lines were afterward rediscovered, in 1814, by a German optician named Fraunhofer, who explored them so carefully that they have since been called after his name. He studied the spectrum, formed by a fine slit, with the telescope, and found that the lines were very numercus, that they varied in thickness and were distributed in unequal groups through the spectrum. He counted 590 from the red to the violet, and made a map of them (Fig. 106), designating the most important by the letters of the alphabet. Fraunhofer further found that the lines did not vary in sunlight, examined at different times; that the reflected light from the moon, or from Venus, gives the same distribution of them as the sun, while the spectra of the fixed stars differ from those of the sun, and from each other. From these considerations