0. A. Young—Spectroscopie Notes. 355 
12-feet equatorial of our observatory, so that it is kept by the 
driving clock directed to the sun. An image of the sun is 
formed on the slit by an achromatic object glass of three inches 
aperture, in order to increase the light and to avoid the widen- 
ing of the lines due to the sun’s rotation. A large prism of 
about 20° angle was sometimes placed in front of this object 
glass (between it and the sun) to separate the colors before 
reaching the slit; and in examining the darker portions of the 
Spectrum a concave cylindrical lens was sometimes used next 
the eye, like a shade glass, to reduce the apparent width of the 
Spectrum and thus increase its brightness. 
e grating is an admirable one, on the whole the best I 
have ever seen. But I have been greatly surprised at its ex- 
cessive sensitiveness to distortion by pressure or inequalities of 
temperature. Although the plate is fully 3 of an inch thick, 
and only 34 inches square, an abnormal pressure of less than 
a single ounce at one corner will materially modify its behavior, 
and a quarter of a pound destroys the definition entirely. 
fact the plate is not naturally exactly flat, and to get its best 
performance it is necessary to crowd a little wedge gently 
under one corner. When it is in good humor and condition, 
however, the performance is admirable; one could wish for 
nothing better, unless for a little more light in the violet por- 
tions of the spectrum. 
With this instrument I have examined the 70 lines given on 
Angstrom’s map as common to two or more substances. 
(At the time of the meeting of the American Association for 
the Advancement of Science, at Boston, I had finished the 
examination of only 47). Of the 70 lines, 56 are distinctly 
double, or triple; 7 appear to be single ; and as to the remain- 
ing 7, I am uncertain; in most cases, because I was unable to 
identify the lines satisfactorily on account of their falling upon 
spaces thickly covered with groups of fine lines, none of which 
are specially prominent. ; 
eneral rule the double lines are pretty close, the dis- 
tance being less than that of the components of the 1474 line. 
Generally also the components are unequal in width or dark- 
ness or both, though in perhaps a quarter of the cases they are 
alike in appearance. The doubtful_lines are the following, 
designated by their wave length on Angstrom’s map: 5489-2, 
54250, 5896-1, 5265°8, 4271°5, 4253-9 and 42268. I strongly 
suspect 5396°1 and 5265°8 (which present no difficulty in iden- 
tification), of being double, but could never fairly split either 
of them, and therefore leave them among the doubtfuls. 
Those which show no signs of doubling, so far as could be 
pie were: 6121-2, 6064°5, 5019-4, 4585-8, 4578°3, 4249°8, and 
5. 
