Spectrum analysis : six lectures, delivered in 1868, before the Society of Apothecaries of London / by Henry E. Roscoe ; with appendices, coloured plates, and illustrations.

Date:
1869
Catalogue details

Licence: Public Domain Mark

Credit: Spectrum analysis : six lectures, delivered in 1868, before the Society of Apothecaries of London / by Henry E. Roscoe ; with appendices, coloured plates, and illustrations. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    it tn be possible, by receding from tbe spark, to find a position in wliicli tbe double line in the green, with which the line in the nebula coincides, was alone visible, and the spectrum of the spark in nitrogen resembled that of a monochromatic nebula. It is obvious that, if the spectrum of hydi’ogen were reduced in intensity, the line in the blue, which corresponds to that in the nebula, would remain visible after the line in the red and the lines more refrangible than F had become too feeble to affect the eye. It therefore becomes a question of much interest whether the one, two, or three, or four lines seen in the spectra of these nebulae represent the whole of the light emitted by these bodies, or whether these lines are the strongest lines only of their spectra, which, by reason of their greater intensity, have succeeded in reaching the earth. Since these nebulae are bodies which have a sensible diameter, and in all probability present a continuous luminous surface, or nearly so, we cannot suppose that any lines have been extinguished by the effect of the distance of these objects from us. If we had evidence that the other lines which present them- selves in the spectra of nitrogen and hydrogen were quenched on their way to us, we should have to consider their disappearance as an indication of a power of extinction residing in cosmical space, similar to that which was suggested from theoretical con- siderations by Cheseaux, and was afterwards supported on other grounds by Olbers and the elder Struve. Further, as the lines which we see in the nebulae are precisely those which experiment shows would longest resist extinction, at least so far as respects their power of producing an impression on our visual organs, we might conclude that this absorptive property of space is not elective in its action on light, but is of the character of a general absorption acting equally, or nearly so, on light of every degi'ee of refrangibility. Whatever may be the true state of the case, the result of this re-examination of the spectrum of this nebula appears to give increased probability to the suggestion that followed from my former observations, namely, that the sub- stances hydrogen and nitrogen are the principal constituents of the nebuhe of the class under consideration.