144 S. P. Langley—New method in Solar Spectrum Analysis. 
of disturbance. A touch on the prisms, a movement of the slit, 
an adjustment of the eye-piece, will ordinarily disturb one 
spectrum relatively to the other, by a minute amount. But the 
whole change we are seeking is of a minuter order still; how 
then, can we discriminate it with certainty? In our ability to do 
this lies the advantage of the method I describe, which, grant- 
ing sufficient dispersive power, makes impossible the instru- 
mental error that has been dwelt on by Secchi and others with . 
justice, as shaking confidence in the result. To see how we 
ate authorized to use this word “ impossible,” let us bear in 
mind that the solar spectrum consists of two distinct kinds of 
lines, one caused by absorption in the solar, the other by ab- 
sorption in the terrestrial atmosphere. These latter being 
formed by light from all parts of the sun are independent of its 
rotation. 
spectra, as we see by the opposed chromosphere lines, but as 
— were tangible things, like two engine-divided scales, 
whose 
Moving the instrument 90° more, we come again into the 
axial line of the sun, and the coincidence should return: witb 
still 90° more we are again in the equator, but now spectrum 
A is formed by light from the western edge, and this time it is 
