Light; a course of experimental optics, chiefly with the lantern . any other; so that when Mr. Crookessome years ago, found in the spectrum of some lead-refuse volatilised, a new green line none of the knownmetals had yielded, he knew he had something beforehim hitherto unknown, and pursued his investigation tillhe had separated the new metal Thallium. As in allprevious instances, Light was to him a true Revealer of theunknown. Lithium and Strontium are pretty easily volatilised in theshape of their chlorides, and a small quantity of these salts 134 LIGHT. [chap. will generally show visible li
Light; a course of experimental optics, chiefly with the lantern . any other; so that when Mr. Crookessome years ago, found in the spectrum of some lead-refuse volatilised, a new green line none of the knownmetals had yielded, he knew he had something beforehim hitherto unknown, and pursued his investigation tillhe had separated the new metal Thallium. As in allprevious instances, Light was to him a true Revealer of theunknown. Lithium and Strontium are pretty easily volatilised in theshape of their chlorides, and a small quantity of these salts 134 LIGHT. [chap. will generally show visible line spectra in a good Bunsenburner; if not, they may be carefully held in the oxygen sodium is at once the most striking and most easilymanipulated example for screen work. 80. Reversed Lines.—In 1859 Kirchoff cleared up themystery of the solar spectnim, by ascertaining that whenthe vapour of sodium was interposed between the slit andthe solar spectrum, the d line was darkened. We have seenthe bright i> line of sodium, as projected from the Fig. 87.—Reversed ScJdium Line. We now restore the lime cylinder to its place, and throw the--continuous spectrum on the screen in the ordinary way. 1 With the table spectroscope, where much less illuminating power isrequired, many substances may be used which are useless with thelantern. With the latter, nothing beyond absorption spectra and theline spectrum of sodium can be shown without the lime-light. ^ Nothing less than ,the liine-light is brilliant enough to show thereversed lihe on the screen. In private experiments with a prjsm, agood paraffin lamp is bright enough to show reversed lines with manychlorides. VII.] REVERSED LINES. 135 As near the slit as possible we adjust the Bunsen burner,between the sht and the lens; and between the burner andthe lens, again, we interpose a screen of black tin or blackcard, with a somewhat larger slit, so as to cut off the diffusedlight from the sodium from the screen. Intro
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