12 ROBERT M. YERKE8 AND JOHN B. WATSON 



tungsten lamp as a source/ is recommended for use in connec- 

 tion with the methods of investigating vision whose adoption, 

 subject to all possible improvement, we urge. 



The Hefner amyl-acetate lamp provides a photometric stan- 

 dard which is unsatisfactory because of its yellowish color. 

 In all cases, qualitative (color) difference between the stan- 

 dard and the photometered light should be avoided. 



If an electric standard is inaccessible, a standard candle may 

 be employed. 



In case the Lummer-Brodhun photometer is not available, 

 some cheaper form, (for example, the Bunsen) ^ may, with 

 care, be used to advantage. It is, however, high time that we 

 students of animal behavior realized that the quantitative 

 investigation of a problem can be made worth while only by the 

 employment of the best methods of measurement which are 

 available. The emplo3^ment of the most precise instruments 

 for the investigation of vision can not be too strongly urged. 



For the determination of extremely low intensity values 

 (threshold stimuli) in limited spaces, we recommend that the 

 Lummer-Brodhun screen be used in connection with a rotating 

 sector. By means of the latter the intensity of the standard 

 may be reduced to any desired amount. 



We further recommend that the values of lamps be given in 

 candle power (c. p.) and the value of stimuli in candle meters 

 (cd. m.). 



b. Radiometry of white light stiuuili. — " Light is not a simple 

 multiple of radiation, but a complicated function of its quality, 

 quantity, and duration." ^ For this reason, measurements of 

 the energy of light are of uncertain value in physiology and 



^ Standardized electric lamps may be purchased of the General Electric Com- 

 pany, Harrison, New Jersey, or of the Electrical Testing Laboratories, SOth Street 

 and East End Avenue, New York, and the latter will make special photometric 

 determinations, at rates ol)tainable on request. Prolilems relating to units of 

 measurements and standards may advantageously be referred Ijy the inexperienced 

 investigator to the National Bureau of Standards, Washington, D. C, S. W. Strat- 

 ton. Director. 



2 For an excellent discussion of " photometric units and standards " and " the 

 measurement of light " the reader is referred to Lectures on illuminating engineering, 

 Baltimore, The Johns Hopkins Press, 1911, vol. 1, pp. 387-506. These two vol- 

 umes of lectures present also invaluable information concerning the characteristics 

 of sources of light. 



^Nutting, P. G. The Luminous equivalent of radiation: Bull. U. S. Bureau of 

 Standards, 1908, vol. 5, p. 262. 



