10 ROBERT M. YERKES AND JOHN B. WATSON 



i. Recommendations concerning sources for tests of light 

 vision. — In view of the above considerations, and a multitude 

 of others which space limitations force us to omit, we offer the 

 following recommendations concerning sources of stimuli for 

 the investigation of light vision. 



(i) That for all strictly accurate quantitative work the tungs- 

 ten, or the Nernst, be used, in its mid -life period, and on a well- 

 regulated circuit. 



The lamps should be burned on a rheostat circuit at a voltage 

 slightly below the optimal (2-3 volts for no volt direct current 

 lamp), so that as the efficiency diminishes with age (as is shown 

 by frequent photometric tests), an increase in voltage will serve 

 to maintain the lamp at its standard efficiency. This precau- 

 tion, and method of obtaining a constant intensity, is especially 

 advantageous in connection with experiments which must be 

 continued for weeks and in the midst of which it is inadvisable 

 to change lamps. 



Attention is called to the following important points in con- 

 nection with the use of tungsten and Nernst lamps, (a) The 

 lamp should be well seasoned and in perfect condition; (b) it 

 should have been burned under constant pressure for from ten 

 to one hundred hours, according to the type, before being em- 

 ployed in quantitative experiments; (c) the current should be 

 carefully regulated during use of lamp; (d) by frequent metric 

 tests the experimenter should discover variations in the quaHty 

 or intensity of the light yielded by a lamp, and he should replace 

 it as soon as he detects marked changes, such for example as 

 those indicating the end of life. 



(2) That the sun be employed as a source for check or com- 

 parison experiments, in order that the normal reaction of the 

 animal shall be observed with certainty. That experiments 

 with sunlight or daylight be performed at the same hour each 

 day, preferably toward noon, and under as nearly identical 

 atmospheric conditions as are obtainable. 



(3) That in case of the inaccessibiHty of an electric current, 

 acetylene or gas be used under carefully controlled conditions. 



(4) That for qualitative, as distinguished from quantitative, 

 experiments, sunlight or daylight be employed in the study 

 of light vision. 



