442 ME. J. N. LOCKYEE ON SPECTEOSCOPIC OBSEEVATIONS OP THE SUN. 
the moon, in which atmosphere the vapours which give rise to the selective absorption 
of the photospheric light are situated at various heights according to their vapour-den- 
sity, writes as follows : — 
“ Directly outside the photosphere there lies a stratum of the sun’s atmosphere which 
is still hotter than the photosphere, and on the outer boundary of this hot region there 
appears to be a shell of excessively faint cloud, part of which is seen in Mr. De La Rue’s 
photographs of the eclipse of 1860. It probably extends the whole way round the sun. 
It is therefore very desirable that this faint shell, which seems to lie at a distance of 8" 
or 10" from the edge of the sun’s disk, should be observed, both from a central station 
and from stations close to the northern and southern limits of totality, so as to ascertain 
whether we have reason to presume it is continuous round the disk”*. 
Broadly, then, up to the spectroscopic determination of its continuity and real nature, 
the story of the chromosphere is as follows : — 
Arago refers it to clouds at the base of the Corona, and regards these clouds 
as the origin of non-nucleated spots, the dimming of the limb being due to 
the Corona. 
Grant acknowledges the continuity of the envelope, ascribes its brilliancy to 
reflected light, and thinks it may be due to the “cloudy stratum” being 
driven by convection-currents through the photosphere. 
Swan terms it a new envelope of the sun, states that it shines by reflected light, 
and ascribes to it the dimming of the limb. 
Littrow describes it as a continuous thin envelope. 
Liais describes it as a continuous envelope, and gives its thickness as 3" # 3. 
Le Verrier describes it as the unique atmosphere of a solid sun and the origin 
of all spots. 
Secchi acknowledges it as a third envelope. 
Stoney considers it as a stratum of cloud at a distance of 8" or 10" from the 
photosphere. 
Description op the Plates. 
PLATE XXXVII. 
Figs. 1, 2, and 3 show the position of the lines observed on October 20, 1868, and 
their usual form, i. e. the line F is broad at the base and gradually tapers upwards, 
while C, and the line near D (with no corresponding absorption-line ordinarily visible) 
do not, as a rule, present this peculiarity with the instrument employed. 
Fig. 4 shows the appearance of the F line observed on the 5th of November 1868. 
Figs. 5 and 6 show the appearances of the C and F lines observed with a tangential slit ; 
* Monthly Notices of the Eoyal Astronomical Society, vol. xxviii. p. 19. 
1842. 
1842. 
1851. 
1851. 
1858. 
1860. 
1860. 
1867. 
