METHODS OF STUDYING VISION IN ANIMALS 5 



now examine the most important and widely available natural 

 and artificial sources of light. 



a. The sun. — For the eyes of most animals, it is well known 

 that daylight and sunlight are the natural photic stimuli. In- 

 deed, it is in connection with the reception of such stimuli that 

 the eye has developed. As pertinent evidence of the truth of 

 this statement we may mention the fact that the maximum of 

 the luminosity curve for the human eye and that of the energy 

 curve of sunlight lie in the same region of the spectrum. 



The sun fulfills neither the second nor the third of our four 

 requirements : it is uncontrollable experimentally and incon- 

 stant. To a degree which few untrained observers realize, the 

 composition or color values of daylight and sunlight vary with 

 the condition of the atmosphere, the altitude of the observer, 

 the hour, and the season. Like statements may be made con- 

 cerning intensity, for at mid-day the light is many times more 

 intense than at morning or evening, and during the summer 

 it is likely to be more intense than during the winter. These 

 considerable variations in quality and intensity, in connection 

 with the fact that no human experimenter is able to control 

 them accurately, render the sun unsatisfactory as a source of 

 stimuli for the qiiantitive study of light vision. For qualitative 

 investigations it is invaluable because of its naturalness. 



In naturalness the sun is preferable to all other sources of 

 stimuli; with respect to controllaVjility and constancy it is 

 surpassed by several artificial sources ; and in measurability 

 it is as satisfactory as most other sources. If we could devise 

 satisfactory ways of controlling the quality and intensity of 

 sunlight, it obviously would be ideal for the study of vision. 



h. Ncrnst lamps. — The Nernst glower yields a light which is 

 qualitatively and intensively fairly satisfactory for the study 

 of vision. In quality its light, although less like daylight than 

 that of the open arc, is about as satisfactory, all things considered, 

 as that of any artificial source at present available.' It, there- 

 fore is likely to prove reasonably natural. Its spectrum is con- 

 tinuous, but rather weak in the violet. 



The quality and intensity of the Nernst light is to a high 



^ Progress in the art of artificial illumination is so rapid that what is written 

 to-day may be misleading to-morrow. The statements in this report must be 

 read with the understanding that they were true in the year 1910. 



