C. A. Young—wSpectroscopic Notes. 357 
Let angle between telescope and collimator = a. 
ngle between telescope and normal to grating = t. 
Then angle between collimator and normal = % = a — r. 
Also, let space between adjacent lines « of os = 5, 
And the order of spectrum observe 
Then, by principles of spectrum formation we have 
A=} sine — sin xl, 
nr 
4 being the ae Abo of the ray which is in the center of 
the field of v 
whence mn t= sect sin 4. 
s 
Differentiating, we have at once 
COS H cn om 
dt =e du, or praia 
which reduces to, dr = (cosa + sina tant)dx. Distortion 
can only disappear i in cases when this coefficient of dx reduces 
to unity. Special cases— 
1. If t= there is no distortion—but also no dispersion : 
it is the case of simple reflection. 
2. If x=0, the grating being kept normal to the collimator, 
it ween en adn. 
If t=0, the grating being kept normal to the telescope 
(which i in this case we pe ina then dr = cos a dx. 
4. Ifa pened: ae = = 
the eyepiece. An instrument on this plan is ee made for 
Profedape Brackett i the Clarks, for use in the physical labora- 
tory at Princeton, and is now nearly completed. 
Princeton, Sept. 27, 1880 
Note to the ehh Article, by Professor C. A. Youne.—An 
observation made since my p paper was written, leads me to modify 
this opinion, that he companion of H is due to byamgen and 
e hydrogen-lines, At 11 a. m. on October 7, a bright horn ap- 
peared on the S. E. limb of ion un. When firs st seen it was about 
3’ or 4’ in elevation, but it rapidly stretched up and before noon 
reached a measured altitude of over 13’ (350,000 miles +) above 
