OF SPECTRA AT THE SOLAR ECLIPSE OF JANUARY 22, 1898. 
O Q O 
OOO 
nient in order to obtain a good focus over a long range of spectrum, and the slight 
shift of the whole spectrum in the direction of its length, which occurred when the 
chromosphere arcs changed from the east side of the sun to the west, was sufficieiit to 
spoil the focus for most of the spectrum, and shift the region of good focus further 
towards the more refrangible end tlian had been allowed for. 
It was found possible, however, to make fairly good measures of all the lines shown 
in No. 7 spectrum. 
Pliotographs oj the Cusp before Totality. 
Spectra Nos. 1 and 2 (Plate 10, fig. I).— The first two photographs of the series are 
mages of the cusp spectrum, the exposures being made 20 and 10 seconds before 
totality respectively. At the moment when the first exposure was made, the strip of 
photosphere still uncovered had a width of 8" at the centre of tlie cusp, and this 
had diminished to 4" only when No. 2 was exposed. The cus]) therefore acted the 
part of an exceedingly tine slit, and gave beautiful images of the Fraunhofer lines. 
The two images olhained may be considered together as they are alike in every 
respect, excepting in the width of the continuous spectrum. This is 5'20 millims. in 
No. 1, and 4‘78 millims. in No. 2, the moon’s diameter on the plate measuring 8‘45 
millims. The continuous spectrum shows most of the features of the ordinary Fraun¬ 
hofer spectrum, the dark lines being rejjresented by very sliarply dehned curved arcs. 
These are, however, very much less dark than the lines in the spectrum of ordinary 
sunlight, and the hydrogen lines /3, y, and S are not present as dark lines at all. 
H and K are broad and dark, with an irregular reversal at one point showing motion 
in the line of sight. 
With the exceptioji of the al^sent hydrogen lines the relative intensities of the 
dark lines in these spectra appear to l^e identically the same as in the Fraunhofer 
spectrum throughout the whole region photographed, ljut the cusp spectra have a 
pale “ washed-out ” appearance when placed Ijeside photographs of the spectrum of 
ordinary daylight, and this peculiarity does not seem to be due to any accident of 
exposure or development.^ (Compare the dark lines in figs. 1 and 3, Plate 10, with 
the corresponding lines in fig. 2.) 
* [After returning to England a set of photographs of the Fraunhofer spectrum was obtained with 
the same prismatic camera, for comparison with the cusp spectra. These were obtained in the same 
manner as the preliminary trial plates mentioned above, using a reflecting collimator and a slit. 
In order to reproduce as closely as possible the form of the solar cusp a circular slit was used, this was 
formed by a round hole cut in a plate of brass and nearly covered by a circular disc of slightly larger 
diameter. The hole was made to subtend an angle of 32' at the prisms, and the ratio of the diameters of 
the hole and disc was made the same as that of the sun and moon at the eclipse. 
In making the exposures the slit was simply directed to a clear sky at a considerable altitude. Fig. 2, 
Plate 10, is a reproduction of one of the spectra so obtained, in which the breadth of the band is the same 
as that of the cusp spectrum in fig. 1. The angular width of the slit was therefore the same as that of 
the solar cusp at the moment when the cusp spectrum was photographed (about 8" at the middle point).— 
March 9, 1901.] 
