OF ARTS AND SCIENCES : JANUARY 9, 1866. 63 



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 

 Hi line through an angle of 280° 42' 20", when each passes through 

 the prisms at the angle of minimum deviation. The dispersive power 

 of the instrument for these two rays is therefore 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 dimensions. With a more powerful instrument a 

 larger number of aqueous lines would be seen under the same atmos- 

 pheric conditions. The Cambridge instrument has a set of sulphide of 

 carbon prisms which disperse the light nearly twice as much as the 

 flint prisms. The sulphide of carbon prisms are very variable in their, 

 action ; but, under the best conditions, they might show the D line as 

 in Fig. 3, when with the flint prisms it would appear as in Fig. 2. 



The facts stated in this paper fully account for the discrepancies in 

 the representations which different observers have given of the D line. 

 Some time since, Mr. Gassiot, of London, gave in the Chemical News 

 a representation of the D line as seen with his instrument, showing 

 several lines in addition to those seen by myself and other observers. 

 On visiting the Kew observatory, in the summer of 1864, I was sur- 

 prised to find that this instrument was less powerful than the one I was 

 then using; and I also learned that these lines were only seen on a 

 single occasion. The moist climate of England is the evident expla- 

 nation of the additional lines. 



As I stated at the first of this paper, the D line has been selected 

 simply to illustrate a general truth. The development of aqueous 

 lines in contiguous portions of the spectrum is even more marked than 

 in the exceedingly limited portion here represented. Indeed, as has 

 been already intimated, the number of these lines seen in the yellow 

 region of the spectrum, on the 17th of November, was at least ten 



* Thcsse prisms were furnished by the American Academy from the income of the 

 Rumford Fund, appropriated for investigations on light and heat. See Proceedings 

 of the American Academy, Annual Meeting, May 24th, 1864. 



