TOTAL ECLIPSE OE THE SUH, APRIL 16, 1893. 
599 
Variability of the Continuous Spectrum , 
No data have been recorded in regard to the varying intensity of the so-called 
continuous spectrum of the corona, but, from my spectroscopic observations of 1871, 
1878, and 1882, there is no question whatever in my mind that there is a consider¬ 
able variation in brightness in this continuous spectrum at the maximum and 
minimum sun-spot periods. The absence of definite statements seems to suggest the 
desirability of having this question studied by means of the visibility of stars, and, if 
possible, numerical data should be obtained at each eclipse. 
During the eclipse of 1886 the sun itself was clouded over, as seen from the station 
occupied by myself in the West Indies ; but in other parts of the sky a great number 
of stars was visible—a much greater number than is visible at full moon. At the 
African station in 1893, the corona was so bright that only the planets Jupiter and 
Yenus were seen by Mr. Fowler. 
There is, however, direct evidence of change of the total light of the corona plus 
the prominences and chromosphere from one eclipse to the other; and it is probable 
that the brightness of the so-called continuous spectrum varies in the same way. 
Photometric measurements made in the eclipse of 1870 showed that the total 
light of the corona was represented by 0*42 of a standard candle at a distance of a 
foot.* Similar observations made in 1878 led Professor Harkness to conclude that 
the total light of the corona was 0*072 of the same unit.f 
In 1886 the greatest value registered for the total light was 0*02 of the same unit, 
but in connection with this low estimate it is pointed out that the conditions of 
observation were not so favourable as in 18784 Photometric observations were 
made in 1893, but the results have not yet been published. 
Thus, the greatest brightness of the total light was recorded at a sun-spot 
maximum, while the light was very much less near the times of minima. 
So far as they go, these facts agree with the view that; the true corona is brightest 
near a sun-spot maximum. 
The more solar physics is studied, the more an enormous change from maximum to 
minimum in all the phenomena is revealed. Besides this variability of the corona 
there is, in addition to the well-known eleven-yearly variation in the number of spots, 
faculae, and prominences, a variation in the spectra of sun spots so marked, as I have 
shown in other communications, that there are few, if any, widened lines common to 
the maximum and minimum. 
* ‘U. S. Coast Survey Reports,’ 1870, p. 172. 
t ‘Washington Observations,’ 1876, App. III. 
X ; Phil. Trans.,’ vol. 180, A, p. 381. 
