J. W. Draper—Distribution of Heat in the Spectrum. 169 
‘ 
quity to its incident rays as to throw the focal images suffici- 
ently on one side. Yet this obliquity must not be greater than 
's actually necessary for that purpose, or the purity of the sec- 
ond spectrum, with its Fraunhofer lines, will be interfered with. 
Fig. 3. 
it the place of the third focus, arising from the reunion of the 
Spersed rays, is the thermopile g, connected by its wires kk 
With the multiplier m. ; 
_henever any of the visible rays of the Fraunhofer spectrum 
are intercented by advancing either of the screens A, 2, the 
image on » face of the pile ceases to be white. It becomes of 
cular ray, or of any selected combination of rays. The screens 
can be ar ged oe us to reach any designated Fraunhofer Tine. 
The pile have used is of the common square form; a linear 
Pile would not answer. The focal image on the pile is of very 
