SUNLIGHT AND ITS MEASUREMENT 195 



Because light sources are usually used only to render objects 

 visible, the measurement of their illuminating power in terms 

 of that of some standard source has come to be of importance 

 quite independently of any question of the actual energy emitted 

 b3' these sources. Thus photometry deals with meter candles 

 and lumens as its units instead of employing units of energy. 

 If the light sources are of the same kind so that the quality 

 of radiation is closely similar, the actual energy values may be 

 readily obtained once that of the primary standard is known. 

 If, however, the light sources are different the question of the 

 light quality becomes of great importance if energy statements 

 are desired, because the human eye is not equally sensitive to 

 all visible radiation. Furthermore the sensitivity curve of 

 the human eye changes with changes in the intensity of illumi- 

 nation in such a way that as the intensity of illumination de- 

 creases the maximum sensitivity shifts toward the shorter 

 wave lengths. Besides these difficulties there are the added 

 ones of the influence of the pupil area (changing the amount 

 of light received by the eye), the psychological effects of con- 

 trast, etc., and the physiological state of the observer. These 

 various influences have received detailed consideration and 

 many, devices have been adopted for particular purposes, for 

 which the special texts and journal articles should be consulted. 



One device deserves special mention because, besides being 

 widely used in photometry, it has been suggested for radiometry 

 and actinometry as well — the rotating sectored disk for re- 

 ducing light intensity. Its use is based on Talbot's law 42 that 

 upon intermittent illumination of sufficiently short periods of 

 interruption the human eye integrates the effect by a simple 



42 Talbot, W. F., Experiments on light. Phil. Mag. (3) 5: 321-334. 1834. 



Wiedemann, E., TJeber Fluorescenz und Phosphorescenz. Ann. Physik. 

 Chem. 34: 446-169. 1888. 



Hyde, E. P., Talbot's law as applied to the rotating sectored disk. Bull. 

 U. S. Bur. Standards 2: 1-32. 1906. 



Weher, A. E., liber die Anwendung des rotierenden Sektors zur photogra- 

 phischen Photometrie. Ann. Physik 45: 801-83-i. 1914. 



Pfund, A. H., cited in note 53. 



Newcomer, H. S., The relation of the sector opening of the sector photom- 

 eter to the extinction coefficient. Science (N. S.) 49: 241-243. 1919. 



