194 HOWARD E. PULLING 



with reference to the conditions under which it is proposed 

 to use it. In conclusion, a "thermo-galvanometer" described 

 by Jordan 40 deserves mention as a relatively simple device 

 for some purposes. 



2. Photometry* 1 



Because the human eye is sensitive to differences in illumina- 

 tion it has been utilized as a photo-sensitive instrument, de- 

 fining light, not in energy units, but in units of physiological 

 effect — brightness. For this purpose a light of known intensity 

 is required, which is varied in a knowm way until it produces 

 an illumination that is equal to that produced by the source 

 under investigation. 



40 Jordan, F. W., On a new type of thermo-galvanometer. Proc. Phys- 

 Soc. London 26: 165-171. 1914. 



41 1918 report of the committee on nomenclature and standards of the Ilium i- 

 nating Engineering Society. Trans. Ilium. Eng. Soc. 13: 512-523. 1918. 



Rosa, E. B., Photometric units and nomenclature. Bull. U. S. Bur. Standards 

 6: 543-572. 1909-1910. 



Ott, E., Variation of Hefner lamp with atmospheric conditions. J. Gas-- 

 lighting 132: 378-381. 1915. 



Hyde, E. P., and W. E. Forsythe, Visibility of radiation in red end of visible 

 spectrum. Astrophys. J. 42: 285-293. 1915. 



Blanchard, J., The brightness sensibility of the retina. Phys. Rev. 11: 

 81-99. 1918. 



Coblentz, W. W., and W. B. Emerson, Distribution of energy in the visible 

 spectrum of the acetylene flame. Bull. U. S. Bur. Standards 13: 355-361. 1916. 



Idem. Selective radiation from various substances. Bull. U. S. Bur. Stan- 

 dards 7: 243-294. 1911. 



Palaz, A., A treatise on industrial photometry with special application to 

 electric lighting. Trans. G. W. Patterson. 2nd. ed. New York. 1896. 



Patterson, C. C., Investigations on light standards and the present condition 

 of the high voltage glow lamp. Proc. Inst. Elect. Eng. 38: 271-348. 1907. 



Idem. The proposed international unit of candle power. Proc. Phys. Soc. 

 London 21: 867-883. 1909. 



Troland, L. T., Apparent brightness; its conditions and properties. Trans. 

 Ilium. Eng. Soc. 11: 947-975. 1916. 



Nutting, P. G., Effects of brightness and contrast in vision. Trans. Ilium. 

 Eng. Soc. 11: 939-916. 1916. 



Barrows, W. E., Light, photometry and illumination. New York. 1912. 



Wiekenden, W. E., Illumination and photometry. New York. 1910. 



Ives, H. E., The primary standard of light. Astrophys. J. 36: 322-329. 

 1912. 



