fl8 
PSYCHOLOGY: R. M. YERKES 
similar to the human. That this opinion is erroneous and demands 
correction is proved by the results obtained by the use of spectral stim- 
uli. It is now definitely known, for example, that, among the rodents, 
the mouse, rat, and rabbit well-nigh lack the ability to distinguish colors. 
For them, long wave-lengths are of surprisingly low stimulating value. 
The spectral range of certain day-birds appears to be similar to that 
of man. Among the higher animals, critical observation indicates that 
the cat and dog possess slight power to respond differentially to differ- 
ent wave-lengths. The observations at present available do not justify 
dogmatic statements, but they conclusively prove that all earlier ac- 
counts of color vision in these animals are misleading. 
During the past ten years, I have devoted a large amount of time 
to the development of methods and the accumulation of facts. My 
study of vision in the dancing mouse,^ which went far toward proving 
the absence of color discrimination in that organism, led me to abandon 
the use of stimuli obtained from colored papers, cloths, or the use of 
ray-filters, and to depend wholly upon spectral light. 
At present I have undertaken the study of color vision in the ring- 
dove,^ using the Watson- Yerkes spectral light apparatus with a type 
of reaction-box which was developed in connection with the study of 
the dancing mouse. The method which is employed in connection with 
this apparatus has been called 'the discrimination method.' Watson 
has recently suggested the name 'sensory habit method.' It involves 
the simultaneous presentation to an organism of two stimuli which 
differ definitely and measurably in some respect or respects. It demands 
of the animal that it react differently to the two stimuH; to the one of 
them positively, to the other negatively. Positive reaction is encouraged 
by the reward of food and inappropriate response is discouraged by 
disagreeable electrical stimulation. The essential features of the appa- 
ratus are, first, a home compartment in which the animal receives stimu- 
lation from two adjacent sources. If, in a certain required manner, it 
approaches the stimulus which has been defined as the 'positive,' it is 
permitted to escape thence by way of a narrow passage to a food com- 
partment where it receives its reward. If, on the contrary, it approaches 
the stimulus designated as the 'negative,' it is punished by electrical 
stimulation, is forced to retreat, and then again has the opportunity 
to react to the 'positive' stimulus. 
The sensory habit apparatus, even in its most improved form, is a 
fairly simple mechanism, and the sensory habit method, with the em- 
ployment of reward and punishment as conditions for careful discrimi- 
