METHODS OF STUDYING VISION IN ANIMALS 7 



c. Tungsten lamps. — This form of incandescent lamp yields 

 a light, which, under favorable conditions, is as satisfactory 

 qualitatively as that of the Nernst glower. In naturalness it 

 may therefore be classed with the latter. The lamp is highly 

 efficient and its light reasonably well fulfills the requirements 

 of controllability, constancy, and measurability. The normal 

 life of the tungsten is about eight hundred hours, and during 

 this period the intensity of the light diminishes by eight to ten 

 per cent. This decrease is barely half that of the Nernst. With 

 the aging of its filament, the quality of the tungsten light changes 

 somewhat, but not so markedly, it would seem, as that of the 

 carbon incandescent. The filament is brittle when cold and 

 there is considerable risk of breakage in moving the lamp quickly, 

 but when hot it is tough and flexible and the lamp may be 

 handled safely. 



For the quantitative study of light vision we recommend the 

 tungsten incandescent lamp (i) because it, as nearly as any 

 other reasonably convenient artificial source, meets the require- 

 ments of naturalness, controllability, constancy, and measur- 

 ability, (2) because it is efficient, inexpensive, easily cared for, 

 and adaptable, and (3) because, when carefully seasoned and 

 used, its quality and intensity do not vary greatly during the 

 mid-life period. 



It should be emphasized that, in contrast with the Nernst, 

 the tungsten lamp exhibits a steady decrease in intensity during 

 its life, instead of relatively rapid decrease at the beginning 

 and end of the life-history and constancy during the mid-life 

 period. This fact renders the Nernst lamp preferable to the 

 tungsten for certain pur]:oses. 



Valuable information concerning tungsten lamps,' which are 

 now manufactured in a great variety of forms, may be obtained 

 from the Bulletins of the National Electric Lamp Association, 

 No. 6 A, No. 6B, No. 6D, No. 6E. These Bulletins may be 

 obtained from the Engineering Department of the National 

 Electric Lamp Association, 441 1 Hough Avenue, Cleveland, 

 Ohio. R. W. Hutchinson's " High-efficiency electrical illumi- 

 nants and illumination," New York, 191 1, also is useful. 



d. Carbon incandescents. — Of the numerous forms of carbon 



' The tantalum lamp has seemed to us less satisfactory for our purposes than 

 the tungsten, and detailed consideration of it is therefore omitted. 



