MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 



31 



to coincide with the cross wires in the field of the instru- 

 ment, it was found more satisfactory to determine them 

 by bringing them to one or other of the edges of the 

 field a process by which the extreme rays could be bet- 

 ter ascertained, their faint light being thus more easily 

 perceived in the darkness by which it was surrounded. 

 It would scarcely be possible to see them accurately 

 while the rest of a bright spectrum was in view. 



In Fig. 3, a b is the ignited platinum strip, c the prism, 

 d e the telescope, 

 moving upon the 

 centre of a gradu- 

 ated circular ta- 

 We,// 



As it was ab- 

 solutely necessa- 

 ry to have fixed 

 points of refer- 

 ence, that all the 

 observations 

 might be brought to a common standard of comparison, 

 and as there are no fixed lines in the light of incandes- 

 cence such as are in the sunshine and daylight, I there- 

 fore previously determined the position of the fixed lines 

 in a spectrum formed by a ray of reflected daylight 

 which passed through a fissure -^ of an inch wide, and 

 one inch long, occupying exactly the position subsequent- 

 ly to be occupied by the incandescent platinum. In Fig. 

 4, 1 represents the result.* 



(I expected to use the Fraunhofer lines for this pur- 

 pose, and was not a little surprised to find that they are 

 not to be seen in the spectrum of ignited solid bodies. 



Fig. 3. 



* The letters used to indicate the fixed lines are those employed at that 

 time, 1847. 



