Mr. G. J. Stoney on the Solar Eclipse, August 1868. 505 



Finally, it is evident from the observations of the eclipse of 

 1860, that flame-like protuberances are formed of various ma- 

 terials. Probably they are most of them very attenuated mists 

 of solid or liquid particles ; but it is also possible that there may 

 be some in a gaseous state. It is, in fact, shown in the above- 

 mentioned memoir that if two of the gases of the solar atmo- 

 sphere are kept asunder by the temperatures which prevail in 

 its low-lying strata, but can unite into a compound gas under 

 the diminished temperature and pressure which prevail at greater 

 heights, or vice versa, the gas which in such cases presents itself 

 only at great altitudes will comport itself, in many respects, 

 like a cloud, and in particular will become intensely heated, and 

 emit the rays which constitute its gaseous spectrum. A similar 

 state of things would result from such a change in the spectrum 

 emitted by a constituent of the sun's atmosphere as we know from 

 Pliicker's experiments can take place in nitrogen and other gases. 

 It is desirable, therefore, that the spectra of the protuberances 

 should be examined, if this prove practicable, in order to deter- 

 mine whether they all resemble the solar spectrum, as they must 

 if these clouds are all mists, or some of them consist of bright 

 lines, as they will if gaseous. For such observations a telescope 

 whose motions are under the control of an assistant looking in 

 sideways would probably be best. I think, however, that the 

 persons who are willing to devote themselves to such delicate 

 observations would do well to prepare for them by staying for 

 some time beforehand and up to the commencement of totality 

 in the subdued light of a darkened chamber. 



Shortly after the time of the solar eclipse the morning zodiacal 

 light will be visible in great splendour to the members of the 

 expedition ; and it is very much to be desired that the opportu- 

 nity should not be lost of obtaining a careful spectral examina- 

 tion of it by experts. This would not require any telescope*; 

 but a binocular spectroscope, consisting of two similar instru- 

 ments placed parallel to one another, would double the light f. 



* That is, provided the collimator is sufficiently long to render the an- 

 gular aperture of its lens, viewed from the slit, not more than the angular 

 breadth of the zodiacal light at the part of it observed. Otherwise there 

 will be advantage in placing a lens, which need not be achromatic, in front 

 of each collimator. 



t A similar arrangement might be used with a telescope upon all objects 

 which are not stellar — the two spectroscopes being provided with collima- 

 tors of unequal length and inclined to one another so as to be directed 

 towards two parallel slits brought as close as possible together, and the 

 light being diverted up one of the collimators either by total reflexions, as 

 in binocular microscopes, or by a small achromatized prism. In this way 

 the light would be nearly doubled — an effect which could not be produced 

 by an enlargement of the telescope. Bv moving the achromatized prism 



Phil Mag. S. 4. No. 233. BuppL Vol. 34. 2 L 



