6 ROBERT M. YERKES AND JOHN B. WATSON 



degree controllable by the experimenter. On a well controlled 

 circuit, the life of the Nernst glower is six hundred hours for 

 the direct current and eight hundred hours for the alternating 

 current, regardless of position. During the first tw^o or three 

 hundred hours of the life of a "glower" the intensity of the light 

 yielded by it diminishes rapidly (10-15 P^r cent). A new lamp 

 should be allowed to burn for several hours before being used 

 for quantitative experiments. During the mid-life period of 

 the Nernst it is possible, by accurately regulating the current, 

 to obtain light which is fairly constant in quality and intensity. 

 As a rule it is desirable to mount the Nernst glower separately 

 in order to avoid the influence of the "heating coils" upon the 

 quality of the light. 



Measurements of quality and intensity can be made as readily 

 with this lamp as with any other source of light. The use of 

 from one to six glowers, according to the demands of the in- 

 vestigation, renders intensities of from 50 to 500 c. p. obtainable. 



As compared with the sun, the Nernst lamp is somewhat less 

 satisfactory in respect to the naturalness of its light, vastly 

 more satisfactory with respect to controllability and constancy, 

 and equally satisfactory with respect to measurability. It is, of 

 course, to be noted that it can not be used when high inten- 

 sities of illumination are required. For more than five hundred 

 candle power the sun and the electric arc are more practicable 

 sources of light. 



On the whole the Nernst glower commends itself to us as highly 

 satisfactory for experiments on vision and for the photic stimu- 

 lation of lower organisms. Its adaptability, stability, and 

 efficiency are important considerations. For quantitative work 

 on vision, with stimuli of medium intensities, it certainly is 

 preferable to the sun and to most artificial sources of light. 



For detailed information concerning the performance, char- 

 acteristics of light, prices, etc., of this glower we refer the reader 

 to the Bulletins of the Engineering Department, National 

 Electric Lamp Association of Cleveland, Ohio, to "Lectures on 

 illuminating engineering," Baltimore, The Johns Hopkins Press. 

 191 1, vol. I, p. 72, to "The Standard handbook for electrical 

 engineers" (2d ed., 1908), and to various publications of the 

 Nernst Lamp Company, Pittsburg, Pa. 



