Page:The American Cyclopædia (1879) Volume XV.djvu/251

 SPECTRUM 239 that the beam of light has been spread out by the prisin into a wedge-shaped beam of various colors, which falling on the screen forms a spectrum. This spectrum of the sun's light may be divided into seven colors, red, orange, yellow, green, blue, indigo, and violet, named in the order of their increasing angular deflection from the direction of the beam of light before it encountered the prism. If the light from the flame of a lamp burning alcohol which contains common salt should enter the slit in place of the sun's light, the spectrum formed on the screen will be found on minute examination to consist only of two closely ap- proximate yellow bands, the remainder of the spectrum which had been obtained with the sun's light being entirely absent. With the light from a flame tinged with the vapor of lithium we obtain a spectrum formed only of two bands, one in the red, the other in the orange. It has thus been found that spectra differ widely, according to the nature of the incandescent substances from which they em- anate. Some, as in the case of incandescent solids, like platinum, are continuous and formed of all of the seven colors; others, as in the cases of the spectra of sodium, lithium, and potassium, are formed of colored bands sepa- rated from each other by spaces devoid of all light ; while again other spectra, like those of the sun and of the fixed stars, are continuous, like those of incandescent solids, but crossed transversely by a multitude of very narrow spaces devoid of light, or nearly so. (See SPECTRUM ANALYSIS.) In the present article we shall consider the spectrum of the sun, and will give in order an account of the manner of its production, of the methods of measuring the lengths of the waves of the various rays composing it, and of the actions of light, heat, chemical decomposition, and fluorescence pro- duced by the different spectral rays when they impinge upon bodies peculiarly constituted to develop and make manifest the above named actions. Spec- tra are usually obtained either by the disper- sive action of a prism, or by the diffraction of a " grating " formed by cut- ting with a dia- mond point on glass or on spec- ulum metal sev- eral thousand equidistant and parallel straight lines in the space of an inch. The prismatic spectrum is formed with purity when the sun's light en- ters a fine slit formed between parallel edges about -5*0 of an inch apart, and, after progress- ing into a dark room for 15 or 20 ft., passes 756 VOL. xv. 16 FIG. 1. Fraunhofer's Spectroscope. through a prism of clear homogeneous glass and then traverses an achromatic lens of about 6 ft. focus. This lens is placed so far from a screen that it forms on it the image of ^ the slit through which ' the sunlight enters t HT- prism is replaced by a plane mirror which reflects the rays on to the screen. The prism in the above experiment must be placed at "the angle of minimum devia- tion ;" that is, it must be so adjusted that the incident beam re- ceives the minimum deviation from the refractive action of the prism. Fraunho- fer substituted a tele- scope for the lens and screen, and viewed the spectrum formed at the focus of its ob- ject glass, as shown in fig. 1. This instru- ment is called a spec- troscope. A spec- trum, formed as just described, is crossed transversely by dark lines of various breadths and degrees of blackness. These lines are unevenly dis- tributed throughout the length of the spectrum ; but the same line always oc- cupies the same posi- tion when referred to the tint in which it exists. Fig. 2 gives the spectral lines as mapped by Fraunho^ fer in vol. iv. of the " Memoirs " of the academy of Munich for 1814-'! 5. To dis- tinguish these lines Fraunhofer designa- ted them by the let- ters of the alphabet, in proceeding from the red to the violet end of the spectrum. Thus A exists in the extreme red, while H is in the violet near the boundary of the visible spectrum. Fraun- hofer mapped in the spectrum 576 lines, and ever since the publication of his drawing these FIG. 2. Fraunhofer's Solar Spectrum. I