Solar Spectrum caused by the Sun’s rotation. 825 
velocity of light = 186600 English miles per second, according 
to the latest determination of Cornu. Then, by Doppler’s theory, 
U=186600 x a (1) 
If now, in any group or spectrum lines of small extent, I is 
put for the difference of wave length of the extreme lines of 
the group; 4 for the interval between the extreme lines meas- 
ured in micrometer units, and 6 for the displacement as indica- 
ted by the difference between the micrometer readings on a 
reo line when the slit is placed on the eastern and western 
imbs of the sun’s image respectively, we shall have 
a) Oe 
hecho i se 
d\=Ix z and U 186600; x-—, (2) 
where A of course is to be taken as the mean wave length of 
the group, 
Taking Angstrém’s wave lengths, we find for the D group 
U=1903" and for the 1474 group u=36112; which amounts 
to saying that a velocity of 190°3 English miles would displace 
one of the D lines by a space equal to the distance between 
them, and for the other group a.velocity of 861:1 miles would 
be required, : 
From the sun’s known dimensions and period of rotation, 
adopting Faye’s numbers, the equatorial velocity of its surface 
's easily found to be 1248 English miles per second; U, o 
course, ought to come out double this, or 2-496. 
_ The tables need little explanation ; the expression “ grating 
right,” means that the grating was so inclined as to throw the 
reflected image of the slit to that side of the collimator which 
was remote from the eye-telescope. In this position the spec- 
tra were more dispersed, but less satisfactorily defined than 
those of the same order obtained by turning the grating “ left, 
eae 
\ &, toward the eye-telescope. The first column contains 
‘ons of the micrometer head; the difference between these, 
Siven at the bottom of the column is 4. The fifth column con- 
tins the differences between the numbers in the second and 
fourth columns, and their mean, given at the bottom of the 
Column, is 6. 
n the earlier observations of the D group several other lines 
Were observed besides the three given, but they were so faint 
© readings were very discrepant, sometimes to the extent 
