168 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 19 3 



tUniioiistratioiis, somewhat modified, is illustrated schematically in 

 Figure 2. The infra-red radiation emitted by a liot wire is isolated 

 by a solution of iodine in carbon bisulphide, which absorbs the visible 

 but permits tlie infra-red to pass through. When concentrated at 

 the focus of a mirror the invisible radiation ignites a match in a 

 few seconds. 



By such means and others, Tyndall was able to find in what pro- 

 portion the total energy radiated by a hot body is divided between 

 visible and obscure portions. The relation of this proportion to the 

 temperature of the body, accurately fornmlated by the tlieoretical 

 physicists Wien and Planck, is a most valuable tool for both the 

 astronomer and the physicist. Two applications of this idea may be 

 noted here. The first is the method of determining stellar or plan- 

 etary temperatures hj finding the distribution of energy in their 

 spectra. For example, tlie image of a star is formed at the focus of 

 a telescope on a tiny receiver which has been blaclcened in order that 

 it maj'^ absorb all the radiation which falls on it. Delicate electrical 



MATCH 



Figure -. — Diagram of one of TyndaU's domonstrations. A match 

 placed as indicated is quickly ignited by invisible infra-red radiation 

 from an electric lamp, although match and lamp are several yards 

 apart. L, an electric street lamp ; Mi, M^, concave mirrors ; Fj 

 liquid Alter which absorbs visible but transmits infra-red radiation 



devices indicate the slightest change occurring in the temperature 

 of the receiver. Filters of various kinds, corresponding to the iodine 

 solution indicated in Figure 2, permit the separate measurement of 

 visible and invisible radiation and thus the ratio of these quantities 

 is obtained, leading to a knowledge of the temperature. 



A second illustration is connected with recent events of world- 

 wide interest. When Mr. Edison's golden anniversary was cele- 

 brated he once more constructed an electric lamp like his first of 50 

 years ago. The contrast between such early lamps and those in com- 

 mon use now is indeed striking. The improvements have resulted 

 from a knowledge of the distribution of energy between the visible 

 and the obscure parts of the spectrum. This has led to the opera- 

 tion of modern lamps at far higher temperatures than were formerly 

 used, with resultant increase of luminous efficiency. These two 

 lamps which I now turn on are consuming nearly the same power. 

 One is a type in wide use 20 years ago, the other a modern form. 

 It is evident that the brightness of the modern lamp is much greater 

 than that of the old one. 



