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in and near the sun, yet in theories concerning this celestial body 
hardly any attention has been paid to refraction. The study of 
atmospherie refraction had, long since, made us acquainted with the 
laws of curved rays}, but the first important attempt to investigate 
the influence which refraction in the sun itself must have had on 
the course of the rays, which reach our eye, and consequently on 
the optical image we get of it, was made by Dr. A. Scumipr. His 
paper ,Die Strahlenbrechung auf der Sonne; ein geometrischer Bei- 
trag zur Sonnenphysik”*) leads to very remarkable results, and at 
any rate urges the necessity of submitting the existing theories of 
the sun to a severe criticism from this point of view. 
If it is taken for granted that refraction in the solar atmosphere 
must be taken into account, we must also pay attention to those 
special cases in which extraordinary values — great or small — 
of the refractive index occur; in other words, the phenomenon of 
anomalous dispersion must be reckoned with. 
It is my purpose to show that many peculiarities, which have 
been observed at the border of the sun and in the spots, may easily 
be considered as caused by anomalous dispersion. 
It is not difficult to obtain the experimental evidence that the 
index of refraction of sodium vapour for light differing but slightly 
in wavelength from that for the D-lines, is very different from the 
index for the other rays of the spectrum. 
H. BrecqvereL (C. R. 127, p. 399; and 128, p. 145) used for 
the study of the phenomenon Kunpr’s method of crossed prisms, in 
a slightly modified manner. The image of the crater of an arc-light 
was projected on a horizontal slit, placed in the focus of a colli- 
mator-lens. The parallel beam next passed through a sodium flame, 
which BEcQuEREL had succeeded in giving the form of a prism 
with horizontal refracting edge, and was then, through a telescope 
lens, focussed into an image of the horizontal slit, falling exactly 
on the vertical slit of a spectroscope of rather great dispersion. As 
long as the sodium flame was absent, a continuous spectrum was 
seen in the spectroscope, the height of which naturally depended 
on the width of the horizontal slit. When the flame was introduced 
in its proper place, and good care was taken to limit the parallel 
beam by means of an easily adjusted diaphragm, in such a manner 
that only such light could penetrate into the telescope lens as had 
passed a properly prismatical part of the flame, the spectrum clearly 
1) The litterature ou this subject is to be found i. a. in a dissertation by O, WIENER, 
Wied. Ann. 49, p. 105-149, 1893. 
2) Stuttgart, Verlag von J. B. MerzreR 1891. 
