34:8 Prof. Draper on the production of Light hy Heat. 



faint blue rays. In these cases it was not, however, incan- 

 descence bnt phosphorescence that was taking place. I infer 

 then that all solids, and probably melted metals, shine at the 

 same thermometric point. 



The temperature of incandescence seems to be a natural 

 fixed point for the thermometer; and it is very interesting to 

 remark how nearly this point coincides with 1000° of the 

 Fahrenheit thermometer, when Laplace's coefficient for the 

 dilatation of platinum is used. Upon that coefficient the 

 point of incandescence is 1006° F. 



In view of these considerations, and recollecting that the 

 number given by Daniell is 980^, and that of Wedgwood 94-7°, 

 I believe that 977° is not very far from the true temperature 

 at which solids begin to shine. It is to be understood, of 

 course, that this is in a very dark room. 



I pass now to the second proposition. The jays emitted by 

 the incandescent platinum were next received on a flint glass 

 prism, placed so as to give the minimum deviation, and after 

 dispersion viewed in a small telescope. A movement could be 

 given to the telescope, which was read off" on an annexed scale. 

 However, instead of bringing the parts of the spectrum under 

 measurement to coincide with the wires stretched across the 

 field of the instrument, I found it more satisfactory to deter- 

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

 the field ; a process well adapted to ascertain the position of 

 the extreme rays, the faint light of which contrasted well with 

 the darkness by which it was surrounded. They could not 

 have been so accurately seen while the rest of the spectrum 

 was in view, and as it was absolutely necessary to have fixed 

 points of reference, that all the observations might be brought 

 to a common standai'd of comparison, and as there are no 

 fixed lines in the light of incandescence, such as are in the 

 sunshine and daylight, I therefore previously determined the 

 position of the fixed lines in a spectrum formed by a ray of 

 reflected daylight which passed through a fissure ;7^ijth of an 

 inch wide and one inch long, occupying exactly the position 

 subsequently to be occupied by the incandescent platinum. 

 Fig. 1 represents the results of this observation. 



The strip of platinum was now placed in the position of the 

 fissure which had given the spectrum fig. 1, and its tempera- 

 ture was raised by the passage of a voltaic current. Although 

 I could distinctly see the metal when the heat had reached 

 about 1000' by the naked eye, yet the loss of light in passing 

 the prism and telescope was so great that I found it necessary 

 to carry the temperature to 1210° before a satisfactory obser- 

 vation could be made. At this point the spectrum extended 



