196 HOWARD E. PULLING 



summation of the actual amounts of light reaching it during 

 the periods of illumination. This principle seems to be valid 

 in photometrical work and the device has been of great value. 

 Little is known of the applicability of the device to radiometry 

 and actinometry although with some particular instruments it 

 appears to yield reliable data. 



Since photometry depends upon a standard light source it is 

 of paramount importance that the standard chosen shall be 

 exactly reproducible at any future time and shall be sufficiently 

 constant in behavior during the period of each photometric 

 observation. The standard sperm candle formerly used is 

 much too variable in behavior (Coblentz states that an indi- 

 vidual may vary 18 to 20%) and an exacting search has pro- 

 ceeded in the attempt to settle upon some standard that will 

 meet the requirements with sufficient accuracy. The liquid 

 and gas burning lamps are, as a rule, sensitive to prevailing 

 conditions and are troublesome to operate so that the tendency 

 seems to be toward the choice of incandescent electric lamps 

 since for short periods direct current can be rather readily con- 

 trolled. It may be possible to utilize an electric lamp with 

 direct current controlled by some simple rheostat and operated 

 similarly to an optical pyrometer for measuring radiation of 

 high intensity in the visible spectrum. With appropriate color 

 screens and dry cells the instrument might be very compact, 

 convenient and have a great range of application. 



By the use of a spectroscope and suitable accessories a spectro- 

 photometer may be constructed by which the distribution of 

 luminosity in the spectrum of the incident light may be obtained 

 and from this an approximation to the relative distribution of 

 energy. 43 



Recently the attempt has been made to measure light in 

 terms of its luminosity without having recourse to the human 



43 Hartridge, H., An improved spectrophotometer. J. Physiol. 50: 101- 

 113. 1915. 



Nutting, P. G., A pocket spectrophotometer. Bull. U. S. Bur. Standards 

 2: 317-318. 1906. 



Idem. A photometric attachment for spectroscopes. Bull. U. S. Bur. Stand- 

 ards 7: 239-241. 1911. 



