NO. 8 SAMUEL PIERPONT LANGLEY — ABBOT 9 



sion of the features of the (telescopic) inner corona, which were, 

 to repeat: (i) Extraordinary sharpness of filamentary structure; 

 (2) arrangement not radial, or only so in the rudest sense; (3) gen- 

 erally curved, not straight lines ; (4) curved in different directions ; 

 (5) "i^ery bright close to the edge, and fading very rapidly — fading out 

 wholly at from 5 to 10 minutes from it." 



"THE BOLOMETER AND RADIANT ENERGY" 



" Our knowledge of the distribution of heat in the solar spectrum 

 really begins with this century and the elder Herschel 



" No one, so far as I know, has hitherto succeeded in measuring the 

 heat from a diffraction grating except in the gross, .... 



" I have tried at intervals for the past four years to do this, and 

 having long familiarity with the many precautions to be used in deli- 

 cate measures with the thermopile, and a variety of specially sensitive 

 piles, had flattered myself with the hope of succeeding better than my 

 predecessors. I found, however, that though I got results, they were 

 too obscure to be of any great value, and that science possessed no 

 instrument which could deal successfully with quantities of radiant 

 heat so minute. 



" I have entered into these preliminary remarks as an explanation 

 of the necessity for such an instrument as that which I have called 

 the Bolometer (/?oA^, /xerpov), or Actinic Balance, to the cost of whose 

 experimental construction I have meant to devote the sum the Rum- 

 ford Committee did me the honor of proposing that the Academy 

 should appropriate. 



" Impelled by the pressure of this actual necessity, I therefore tried 

 to invent something more sensitive than the thermopile, which should 

 be at the same time equally accurate — which should, I mean, be es- 

 sentially a ' meter ' and not a mere indicator of the presence of feeble 

 radiation. This distinction is a radical one. It is not difficult to make 

 an instrument far more sensitive to radiation than the present, if it 

 is for use as an indicator only ; but what the physicist wants, and 

 what I have consumed nearly a year of experiment in trying to supply, 

 is something more than an indicator — a measurer of radiant energy. 



" The earliest design was to have two strips of thin metal, vir- 

 tually forming arms of a Wheatstone's Bridge, placed side by side in 

 as nearly as possible identical conditions as to environment, of which 

 one could be exposed at pleasure to the source of radiation. As it was 

 warmed by this radiation and its electric resistance proportionally 

 increased over that of the other, this increased resistance to the flow 



