SUNLIGHT AND ITS MEASUREMENT 197 



eye as a receiver. This is termed physical photometry 11 and 

 is accomplished by interposing between a radiometric instru- 

 ment and the light source an absorption screen having a curve 

 of transmissions for the various wave lengths similar to the curves 

 of sensitiveness of the human eye. The measurements of the 

 radiation received by the instrument are thus indicative of 

 the effects upon an "average human eye" in the same position. 



3. Actinometry 



Because some chemical reactions proceed so much more 

 rapidly in light than in darkness and since the rate of such re- 

 actions is in general more rapid the greater the intensity of the 

 illumination, they have been utilized for the measurement of 

 radiation. The elaborate investigations of sunlight by Bunsen 

 and Roscoe 45 by these means, the attempt to relate features 

 of plant growth to sunlight by Wiesner's 46 measurements along 

 the same lines, the simplicity of operation and the growing 

 accessibility of photographic supplies have led to an extensive use 

 of actinometry in various fields of biological research. Be- 

 cause of this popularity actinometry will be considered more in 

 detail than the relative (or actual) value of the results so far ob- 

 tained by its means would justify in a well balanced discussion. 



The generalizations at the basis of actinometry are that the 

 optical principles of radiation hold for those wave lengths af- 

 fecting chemical reactions and that only those waves that are 

 absorbed by the substances concerned can be chemically active. 47 



44 Ives, H. E., Physical photometry. Trans. Ilium. Eng. Soc. 10: 101- 

 112. 1915. 



Coblentz, W. W., The physical photometer in theory and practice. J. 

 Franklin Inst. 180: 335-348. 1915; 181: 233-241. 1916. 



45 Bunsen, R. W., and H. Roscoe, Photochemische Untersuchungen. Ann. 

 Physik Chem. 96: 373-394; 100: 43-88; 481-516; 101: 235-263; 108: 193-273; 117: 

 529-562. Ostwald's Klassiker Nos. 34 and 38. 



46 Wiesner, J., Photometrischen Untersuchungen auf Pflanzen-physiologische 

 Gebiete. Sitz. Math. -Nat. CI. Kgl. Akad. Wien 102, Abth. 1: 291-350. 1893. 



47 Grotthus, T. von, Ueber die chemische Wirksamkeit des Lichtes und der 

 Elecktricitat. Jahresverhl. kurland. Ges. Lit. u. Kunst. 1: 119-189. 1819. 

 Ostwald's Klassiker No. 152. 



