44 ROBERT M. YERKES AND JOHN B. WATSON 



the Chrysoidine sufficiently to just transmit 5790 and 5461, 

 then add Neptune green until the yellow lines disappear. 



Chrysoidine + Eosine transmits 5790. The chrysoidine should 

 be dilute and the eosine added until the green line disappears." 



For additional formulae and a thorough discussion of ray 

 filters the reader is referred to the recent monograph of Nagel^ 

 and to Busck.2 



Except when a light which gi\'es a line spectrum is used for 

 special purposes, as for example in the case of the mercury 

 lamp, it is desirable to use as a source in work with the trans- 

 mission method a white light of fairly high intensity. Sun- 

 light, daylight, Nernst light, tungsten light, and acetylene 

 light are likely to prove satisfactory. The sun is to be preferred 

 as a source when naturalness is the chief desideratum ; the Nernst 

 or tungsten lamp, when constancy of intensity is important. 

 The transmission method of obtaining chromatic stimuli is 

 decidedly superior to the reflection method in that it permits 

 (a) of the use of a greater variety of stimuli, (b) of the control 

 of the quality and intensity of the stimuli to a greater extent, 

 (c) of more accurate determinations of the wave-length used, 

 and finally, (d) of the spectrophotometric measurement of 

 intensity in a fairly satisfactory manner. On account of the 

 unequal transmission of the infra-red rays by filters, radio- 

 metric measurements are of little value. 



Dry filters are convenient for a great variety of experiments 

 which demand neither stimuli of a single or closely restricted 

 wave-length, nor extreme accuracy of measurement. 



c. Dispersion method. — White light may, by various means, 

 be resolved into a spectrum,^ from which the experimenter 

 may select and isolate light of the particular wave-length he 

 desires to use as a stimulus. 



We may mention, as especially convenient for use in studies 

 of color vision, two mechanisms for the production of spectra: 

 the diffraction grating and the prism. 



Grating spectra are excellent for many of the in\'estigations 



^ Nagel, W. A. Methoden ziir Erforschung des Licht und Farbensinn.s. Tiger- 

 stedt's Handbuch der physiologischen Methodik. S. 43-55. 



Nagel, W. A. Ueber flussige Strahlenfilter. Biologisches Centralblatt, 1898, 

 Bd. 18, S. G55. 



' Busck, G. Ueber farbige Lichtfilter. Zeitschr. f. Psychologie und Physiol, 

 d. Sinnesorgane, 1904, Bd. 37, S. 104-111. 



^ Baly, E. C. C. Spectroscopy. London, 1905 



