182 J. P. Cooke on Aqueous Lines of the Solar Spectrum. 
higher elevation and the light is reflected under less favorable 
conditions. 
In the examples cited above the comparisons were made 
under as nearly as possible the same conditions so as to elim 
nate all causes of variation except the one under consideration. 
Days were selected when the atmosphere was perfectly clear 
and the sun’s light, so far as I could judge, equally brilliant. 
Moreover, the position of the spectroscope and mirror remain 
unchanged during the whole time. This mirror which is use 
for reflecting the sun’s light upon the slit of the spectroscope 18 
so arranged that it can be turned into any position by the ob- 
server while his eye is at the eye-piece of the spectroscope, and 
it was always carefully adjusted to the position of best defint- 
tion at each observation. The manipulation of the mirror 18 
fully as important in the use of the spectroscope as it is im 
ee 
It will be of course understood that the power of developing 
these faint aqueous lines depends very greatly on the optica 
capabilities of the spectroscope, and that the figures here given 
are relative to the instrument used in the observations. This 
instrument has been fully described in the article already cited. 
It is sufficient for the present purpose to state that it is provided 
with nine flint glass prisms’ of 45° refracting angle, which ben¢ 
the rays of light corresponding to the D line through an angle 
of 267° 37’ 50", and that corresponding to the H, line through 
an angle of 260° 42’ 20", when each passes through the prisms 
at the angle of minimum of deviation. The dispersive poweh 
therefore, of the instrument for these two rays is equal to 
13° 4’ 30”, and the rays corresponding to the two D lines aré 
separated 1/10’. The object glasses of the two telescopes of 
this spectroscope are 2} inches in diameter and have 4 focal 
length of 154 inches, and lastly the size of the prisms an of 
the various parts of the instrument is adapted to these dime: 
