94 H.. Draper—Discovery of Oxygen in the Sun. 
hydrogen tubes gave a spectrum photograph of fifteen lines, of 
which only three belonged to hydrogen. In order to be sure 
that none of these were new hydrogen lines it was necessary to 
try tubes of various makers, to prepare pure hydrogen and 
employ that, to examine the spectrum of water, and finally to 
and nitrogen were made, because these gases seemed to be of 
greatest astronomical importance on account of their relation to 
stars, nebule and comets. Before the subject of comparison 
spectra of the sun was carefully examined, there was some con- 
fusion in the results, but by using hydrogen the source of these 
errors was found out. : 
But in attempting to make a prolonged research in this 
direction it soon appeared that it was essential to be able to 
control the electrical current with precision, both as to quan- 
was the first person in America to procure a Gramme machine. 
e was also the first to use a Brayton engine to drive @ 
ramme. 
The dynamo-electric machine selected is one of Gramme’s 
Spbpae made in Paris and is a double light machine, that is, it 
as two sets of brushes, and is wound with wire of such a size 
as to give a current of sufficient intensity for my purposes. — 
It is nominally a 350 candle-light machine, but the current 
varies in proportion to the rate of rotation, and I have also 
modified it by changing the interior ti The machine 
can produce as a maximum a light equal to 500 standard can- 
dles, or by slowing the rotation of the bobbin the current 
may be made as feeble as that of the weakest battery. In 
ono use it is sometimes doing the work of more than fifty — 
rge Grove nitric acid cells and sometimes the work of a single 
mee. 
The Gramme machine could not be used to work an induc- 
