JOHN WILLIAM DKAI'KR. 



certain position with respect to the observer and only shows him 

 objects for which it lias been adjusted, and those in an unsatisfactory 

 manner; but if he moves to one side or to the other, or endeavors to 

 see objects which are not directly in his way, his view is intercepted, 

 or, perhaps, unless he makes a new adjustment, the light is shut out 

 altogether." 



It is, however, by his researches upon Radiant Energy that John 

 W. Draper is best known in the world of science. In an able 

 memoir on the Production of Light by Heat, published in 1847, 

 he described some ingenious experiments with incandescent platinum, 

 by which he conclusively proved : First, that the temperature at 

 which this metal became incandescent was fixed and constant at 

 977 F. ; and, second, that various other substances, such as brass, 

 antimony, gas carbon, lead, and the like, all became luminous at the 

 same temperature, which was that of the gun barrel in which they 

 were inclosed. By means of an extemporized spectroscope he ex- 

 amined the platinum as its temperature was gradually raised by the 

 electric current and showed that as this temperature rose from 

 1,095 to 2,130 F. the spectrum gradually extended itself toward 

 the violet; rays of an increasing refrangibility being successively 

 produced, the frequency of the vibrations increasing with the tem- 

 perature. In order to bring these observations to a common standard 

 of comparison Dr. Draper originated the simple device of using the 

 sun spectrum with its fixed lines as a reference-spectrum, daylight 

 being reflected to the prism through a narrow aperture occupying 

 exactly the position in which the platinum strip was to be subse- 

 quently placed. The relation of the temperature of incandescent 

 platinum to the amount of light emitted by it was also examined, 

 and it was found, photometrically, that at 2,590 F. the light emitted 

 was more than thirty-six times as much as that given out by the same 

 strip of metal at 1,900. Moreover, on measuring, by means of the 

 thermo-pile, the total energy radiated from the incandescent strip, 

 he found that if he took the heat radiated at 980 as unity, that 

 emitted at 2,360 was 17.8; the heat radiated between 1,000 and 

 1,300 being nearly the same in amount as that radiated in passing 

 from common temperatures up to 1,000. In closing this memoir 

 the author says : *' The preceding experiments furnish an easy means 

 of supplying * * * 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. 



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