MI.M..IU I.] TI1K KADIAT1UNS OF IGNITKI) I'.oDIES. 45 



sil)le to rays not comprehended within certain limits of 

 refrangibility. In these experiments it is requisite to 

 raise the temperature of the platinum almost to 1000 

 before we can discover the first traces of light. Meas- 

 ures obtained under such circumstances are dependent 

 on the physiological action of the visual organ itself, and 

 hence their analogy with those obtained by the thermom- 

 eter becomes more striking, because we should scarcely 

 have anticipated that it could be so complete. 



Among writers on Optics it has been a desideratum to 

 obtain an artificial light of standard brilliancy. The 

 preceding experiments furnish an easy means of supply- 

 ing that want, and give us what might be termed a " unit 

 lamp." A surface of platinum of standard dimensions, 

 raised to a standard temperature by a voltaic current, 

 will always emit a constant light. A strip of that metal, 

 one inch long and -fa of an inch wide, connected with a 

 lever by which its expansion might be measured, would 

 yield at 2000 a light suitable for most purposes. More- 

 over, it would be very easy to form from it a photome- 

 ter by screening portions of the shining surface. An in- 

 genious artist would have very little difficulty, by taking 

 advantage of the movements of the lever, in making a 

 self-acting apparatus in which the platinum should be 

 maintained at a uniform temperature, notwithstanding 

 any change taking place in the voltaic current. 



UNIVERSITY OF NEW YORK, Feb. 27, 1 847. 



NOTE. The experiments related in the foregoing pages 

 were made by me between 1844 and 1847. They were 

 published in May in the latter year. 



In the following July, M. Melloni, who was at that 

 time recognized as the chief authority on the subject of 



