1883.] on some of the Questions involved in Solar Physics. 317 



glass, which, as Professor Stokes had shown, produced this effect 

 without interference with the yellow aud other luminous and intense 



I heat rajs. He next endeavoured to estimate the solar temperature 



I by instituting a comj)arison between the spectra due to different 

 known luminous intensities. Starting with the researches of Pro- 

 fessor Tyndall on radiant energy, supplementing them by experiments 

 of his own on electric arcs of great power, and calling to his aid 

 Professor Langley, of the Alleghany Observatory, to produce for him 

 a complete spectrum of an Argand burner, he concluded that with 



j the temperature of a radiant source, the proportion of luminous rays 

 increased in a certain ratio ; whereas in an Argand gas burner only 

 2J per cent, of the rays emitted were luminous and mostly red and 

 yellow, the most brilliant portion of a gas flame emitted 4 per cent., 

 as shown by Tyndall, the carbon thread of an incandescent electric 

 light between 5 and 6 per cent., a small electric arc 10 per cent., 

 and in a powerful 5000-candle electric arc as much as 25 per cent, 

 of the total radiation was of the luminous kind. Professor Langley, 

 in taking his photometer and bolometer up the Whitley mountains, 

 18,000 feet high, had proved that of the solar energy not more than 

 25 per cent, was luminous, and that the loss of solar energy sustained 

 between our atmosphere and the sun was chiefly of the ultra-violet 

 kind. These rays, if they penetrated our atmosphere, would render 

 vegetation impossible, as proved by the lecturer's own experiments 

 above referred to. It was thus shown that the temperature of the 

 solar photosphere could not materially exceed that of a powerful 

 electric arc, or, indeed, of the furnaces previously alluded to, leading 

 him to the conclusion already foreshadowed by Sainte Claire Deville, 



. and accepted by Sir William Thomson, that the solar temperature 

 could not exceed 3000° C. The energy emitted from a source much 

 exceeding this limit would no longer be luminous, but consist mainly 

 of ultra-violet rays, rendering the sun invisible, but scorching and 

 destructive of all life. The accompanying diagram (Fig. 1) of the 

 spectra alluded to shows clearly the gradual advance of the luminous 



jiband, as marked by the letters A to H. 



Not satisfied with these inferential proofs, the lecturer had endea- 

 voured to establish a definite ratio between temperature and radiation, 



j which formed the subject of a very recent communication to the Eoyal 



'Society.* The experiment consisted in heating, by means of an electric 

 current, a platinum or iridio-platinum wire, a metre long, and sus- 

 pended between binding screws, as shown in the accompanying 

 sketch (Fig. 2) ; the energy of the current was measured by two instru- 

 ments — an electro-dynamometer, giving it in amperes, and a galvano- 

 meter of high resistance giving the electro-motive force between 

 the same points in volts. The product of the two readings gave 

 the volt-amperes, or Watts of energy communicated to the wire, and 

 dispersed from it by radiation and convection. A reference to the 



' Proc. of the Eoyal Society,' vol. xxxv. p. 166. 



