320 
CAPTAIN L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUNDER 
designed for measuring the Transit of Venus photographs, and consists of two parallel 
micrometers, rigidly fixed together and capable of sliding in one dmection. With one 
micrometer the image is observed, and with the other the distance is read off on a 
glass scale. 
Two of the images of all the prominences are especially dense, and from their 
position, and by reference to previous results, these can be recognised without much 
doubt as being due to the Solar H and K lines. Taking these as a basis, three other 
lines, h, H (y), and F, can be recognised in the images of prominences I. and II. with 
some degree of certainty. The accuracy of this assumption can best be tested by 
seeing how nearly the wave-lengths of three of these five lines agree with the wave¬ 
lengths of three known lines, on the assumption that the two others are known. For 
this purpose F and li were taken as known with wa.ve-lengths 4860'5 and 4101’2 
respectively. Then the wave-lengths of tlie other lines were calculated on the 
assumption that the distances measured on the plate varied inversely as the square of 
the wave-length. This last assumption is, however, known to be inaccurate, and a 
correction had to be found and applied. This was done in the following manner :— 
A photograph of the Solar spectrum was obtained with the prismatic camera, but 
placing a slit and collimator in front of the prism. The distances apart of known 
lines of wave-length as near as possible the same as those believed to have been found 
in the spectrum of the prominences were measured ; the wave-lengths of these lines 
were then calculated as before on the assumption that the same two lines were known. 
We, therefore, get in this Solar spectrum the known and the calculated wave-lengths 
of certain lines ; the differences between the two give corrections which, if applied to 
the calculated wave-lengths of the lines of the prominences for the same part of the 
spectrum, should give their true wave-lengths. The results thus obtained are 
recorded in the followina' Table :— 
Wave-lengtli obt£ 
ab( 
Prominence I. 
lined as described 
)ve. 
Prominence I 
Order of 
intensity, both 
prominences. 
Names and wave-1 engtlis, 
known lines 
for comparison. 
4860-5 
4860-5 
3 
4860-5 F 
4471 0 
5 
4471-0 f 
43.39-3 
4340-3 
o 
4340-3 H ( 7 ) 
4101-2 
4101-2 
4 
4101-2 h 
8966-3 
3965-0 
2 
3968-5 H 
3932-8 
8930-5 
i 
3932-8 K 
3890-0 
3890-0 
3 
? 3887-0 H (t) 
1 
1 
The three other prominences showed only in the H and K lines, and were not, 
tlierefore, measured. 
It has already been explained how it came about that the instrument was shaken 
