Comparison of Sources of Artificial Illumination. 103 



ments throughout its spectrum showed the quality of its light 

 to be identical with that of our standard incandescent lamp. 



It appears then, that the temperature of the carbon filament 

 in the glow lamp of to-day, when heated to the highest degree 

 of incandescence compatible with permanence, agrees very 

 closely with that reached by the light-giving particles of oil* 

 and gas flames when the latter are maintained under the condi- 

 tions of combustion most favorable to the production of light. 



The Lime Light. 



The source of illumination to which our attention was next 

 directed, was a lime-light of the type used in the magic lantern. 

 The oxy-hydrogen flame by means of which the lime cylinder 

 was maintained in incandescence, was fed by oxygen and 

 hydrogen gas obtained by electrolysis. The gas was drawn 

 from large storage reservoirs under constant pressure and 

 neither burner nor cylinder was readjusted during the time 

 occupied by a complete set of observations. It was found that 

 the lime reached its highest temperature very soon after igni- 

 tion and then fell rapidly in brightness to an almost constant 

 condition, in which it could be maintained without further 

 marked change for a considerable length of time. 



During this first period of incandescence the lime-light is 

 very much whiter than in the permanent condition into which 

 it soon sinks. Its color indeed approaches very closely to that 

 of burning magnesium. Table I gives the results of a single set 

 of observations upon the freshly ignited lime, comprising the 

 mean of three readings in each of nine regions of the spectrum. 

 The mean of several complete sets of observations upon the 

 lime when it had reached its permanent state of incandescence 

 are also given, and these last results are compared with those 

 obtained by Pickering by measurement of the spectrum of a 

 similar source of light. 



In these and the following tables the intensity of each region 

 is given in terms of the brightness of the corresponding region 

 in the spectrum of the 16-candle incandescent lamp selected as 

 a standard ; the region of the D line in both spectra being 

 taken as unity. The four regions studied by Pickering do not 

 coincide in wave-length with those selected by us, nor was his 

 standard of comparison an incandescent lamp. The wave- 

 lengths of his positions and the brightness of his standard in 

 those positions, as compared with that of a gas flame, are given 

 in his paper. Now the color of an argand gas flame does not 

 differ materially from that of our standard, and it is therefore 

 a simple matter to bring his observations and our own into 

 direct comparison. 



