of the Solar Spectrum. 341 



impairs the distinctness, that I have generally found that the 

 aqueous lines are seen best when the sun is near the meridian. 

 Hence with an equal amount of moisture in the atmosphere, the 

 late autumn may be a more favourable season for seeing the 

 aqueous lines than the summer ; for not only must the solar 

 rays, when most brilliant at noon, traverse a greater extent of 

 air, but, moreover, the atmosphere at this time is usually clearer, 

 and the reflected beam of light which enters the spectroscope 

 is at times even more brilliant than when the sun attains a 

 higher elevation, and the light is reflected under less favourable 

 conditions. , 



In the examples cited above, the comparisons were made 

 under as nearly as possible the same conditions, so as to elimi- 

 nate all causes of variation except the one under consideration. 

 Days were selected when the afmosphere was perfectly clear 

 and the sun's light, so far as I could judge, equally brilliant. 

 Moreover the position of the spectroscope and mirror remained 

 unchanged during the whole time. This mirror, which is used 

 for reflecting the sun's light upon the slit of the spectroscope, is 

 so arranged that it can be turned into any position by the ob- 

 server while his eye is at the eyepiece of the spectroscope ; and 

 it was always carefully adjusted to the position of best defini- 

 tion at each observation. The manipulation of the mirror is 

 fully as important in the use of the spectroscope as it is in 

 microscopy. 



It will be of course understood that the power of developing 

 these faint aqueous lines depends very greatly on the optical 

 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 bend 

 the rays of light corresponding to the D line through an angle 

 of 267° 37 ; 50", and that corresponding to the H l line through 

 an angle of 260° 42' 20", when each passes through the prisms 

 at the angle of minimum deviation. The dispersive power, 

 therefore, of the instrument for these two rays is equal to 

 13° 4' 30", and the rays corresponding to the two D lines are 

 separated 1' 10". The object-glasses of the two telescopes of 

 this spectroscope are 2\ inches in diameter, and have a focal 

 length of 15^ inches ; and, lastly, the size of the prisms and of 

 the various parts of the instrument is adapted to these dimen- 

 sions. With a more powerful instrument, a larger number of 

 aqueous lines would be seen under the same atmospheric condi- 

 tions. My own instrument has a set of sulphide-of-carbon 

 prisms which disperse the light nearly twice as much as the 

 Phil. Mag. S. 4. Vol. 31. No. 210. May 1866. 2 A 



