in the Invisible Prismatic Spectrum. 175 
tion may be determined by observation; with the eye in the 
case of the visible, with the bolometer in that of the invisible 
ray. 
To illustrate the use of the above described apparatus under | 
Somewhat unfavorable circumstances, let us consider as an 
example the observations of June 13, 1882, which were taken 
Properly directed by the siderostat, the visible Fraunhofer 
line D,, of the third spectruin of the grating, was caused to fa 
Upon the slit S, of the spectro-bolometer. Then, according to 
€ theory of the grating, there passed, through this slit, rays 
having the wave-lengths— 
0-589 (3d spectrum—visibie. ) 
138 480 spectrum— invisible.) 
; 1767 (1st spectrum—invisible.) . 
: The prism having been removed, and the telescope SS 
Into line, an image of §,, of the same size as the slit itself, 
Was formed in the focus of the object lens, and on testing 
2. 
43" . 46° ao 45" 1° 
With the bolometer, whose face was covered with a screen 
| ae centrally with a 2™™ slit, the heat of this image’ 
