NOTES AND APPENDICES. 39 



It is possible to construct intermediate machines by suitably 

 adjusting the lengths and thicknesses of the wire on the 

 coils j but the figures we have just given show within what 

 limits the Gramme machines used for the electric light are 

 comprised. 



NOTE -C. 



THE CRITERIA OF THE ELECTRIC LIGHT. 



WE here extract the following passages from a paper by 

 W. H. Preece, which we find in the Telegraphic Journal of 

 the 1 5th February, 1879 : 



" Heat and light are identical in character, though different 

 in degree; and whenever solid matter is raised to a very 

 high temperature it becomes luminous. The amount of 

 light is dependent upon the height of this temperature ; and 

 it is a very remarkable fact that all solid bodies become self- 

 luminous at the same temperature. This was determined by 

 Daniell to be 980 (F.), by Wedgwood 947, by Draper 977 ; 

 so that we may approximately assume the temperature at 

 which bodies begin to show a dull light to be 1,000 (F.) 

 The intensity of light, however, increases in a greater ratio 

 than the temperature. For instance, platinum at 2,600 (F.) 

 emits forty times more light than at 1,900. Bodies when 

 raised to incandescence pass through all stages of the 

 spectrum : as the temperature increases so does the refran- 

 gibility of the rays of light. Thus, when a body is at a tem- 

 perature of 



250 F., it may be called warm. 

 500 ,, hot. 



1,000 we have the red rays. 



1,200 orange rays. 



1,300 yellow rays. 



1,500 blue rays. 



i,/oo indigo rays. 



2,000 violet rays. 



