SUNLIGHT AND ITS MEASUREMENT 193 



in certain cases if the instrument is so constructed that both 

 vapor and liquid are confined. Freely evaporating surfaces 

 are so readily affected by air currents and the large heat capacity 

 created by a sufficiently large reservoir of liquid that they seem 

 to be of little or no value at present. The question of the greater 

 effectiveness of some wave lengths than others, still further 

 militates against their use as does also the dependence of rate 

 of evaporation upon the temperature of the surface. Until 

 these features have been taken into account all instruments de- 

 pending upon vaporization can only yield results that are open 

 to question. 



Instruments depending upon temperature changes in electrical 

 resistance have been investigated in great detail. Each has its 

 own weaknesses and merits for which the references, especially 

 the excellent comparative discussion by Coblentz, 39 should be 

 consulted. Although none are very easy to operate the pyranom- 

 eter, the Rubens linear thermopile and its modifications and 

 the radiomicrometer should prove useful in biological research. 



Instruments depending upon volume changes of liquids and 

 gases are influenced by the prevailing temperature of the sur- 

 roundings, and are limited in usefulness by the transmission of 

 their containers and the nature of the thermo-sensitive sub- 

 stance employed. They are of course essentially like the Arago- 

 Davy actinometer. One instrument at least of the type de- 

 pending upon temperature effects upon gases — the Nichols 

 radiometer — is, unlike the majority, useful for spectrum work 

 and, although not portable, it is extremely simple of operation 

 and may be built to have a very high degree of sensitiveness. 

 It has been rather thoroughly investigated by Coblentz.' Some 

 of the instruments appear to be still susceptible of improve- 

 ment and those employing thermo-couples could undoubtedly 

 be sufficiently simplified to permit of general use where a high 

 degree of sensitiveness is not required. It may be noted that 

 if an instrument is selected that necessitates a galvanometer, 

 the peculiarities of the latter must be considered separately 



39 Coblentz, W. W., Instruments and methods, etc., I and II, cited in note 36. 



