Six Lectures on Light eBook

This eBook from the Gutenberg Project consists of approximately 228 pages of information about Six Lectures on Light.

Six Lectures on Light eBook

This eBook from the Gutenberg Project consists of approximately 228 pages of information about Six Lectures on Light.
of rose-coloured protuberances, stretching sometimes tens of thousands of miles beyond the dark edge of the moon.  They are described by Vassenius in the ‘Philosophical Transactions’ for 1733; and were probably observed even earlier than this.  In 1842 they attracted great attention, and were then compared to Alpine snow-peaks reddened by the evening sun.  That these prominences are flaming gas, and principally hydrogen gas, was first proved by M. Janssen during an eclipse observed in India, on the 18th of August, 1868.

But the prominences may be rendered visible in sunshine; and for a reason easily understood.  You have seen in these lectures a single prism employed to produce a spectrum, and you have seen a pair of prisms employed.  In the latter case, the dispersed white light, being diffused over about twice the area, had all its colours proportionately diluted.  You have also seen one prism and a pair of prisms employed to produce the bands of incandescent vapours; but here the light of each band, being absolutely monochromatic, was incapable of further dispersion by the second prism, and could not therefore be weakened by such dispersion.

Apply these considerations to the circumsolar region.  The glare of white light round the sun can be dispersed and weakened to any extent, by augmenting the number of prisms; while a monochromatic light, mixed with this glare, and masked by it, would retain its intensity unenfeebled by dispersion.  Upon this consideration has been founded a method of observation, applied independently by M. Janssen in India and by Mr. Lockyer in England, by which the monochromatic bands of the prominences are caused to obtain the mastery, and to appear in broad daylight.  By searching carefully and skilfully round the sun’s rim, Mr. Lockyer has proved these prominences to be mere local juttings from a fiery envelope which entirely clasps the sun, and which he has called the Chromosphere.

It would lead us far beyond the object of these lectures to dwell upon the numerous interesting and important results obtained by Secchi, Respighi, Young, and other distinguished men who have worked at the chemistry of the sun and its appendages.  Nor can I do more at present than make a passing reference to the excellent labours of Dr. Huggins in connexion with the fixed stars, nebulae, and comets.  They, more than any others, illustrate the literal truth of the statement, that the establishment of spectrum analysis, and the explanation of Fraunhofer’s lines, carried with them an immeasurable extension of the chemist’s range.  The truly powerful experiments of Professor Dewar are daily adding to our knowledge, while the refined researches of Capt.  Abney and others are opening new fields of inquiry.  But my object here is to make principles plain, rather than to follow out the details of their illustration.

SUMMARY AND CONCLUSION.

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Six Lectures on Light from Project Gutenberg. Public domain.