VISUAL PERCEPTION OF THE CHICK 19 



points out that my discussion of controls (2) is ambiguous 

 through my synonymous use of the words "intensity" and 

 "brightness," The luminous intensity of a source, he declares, 

 involves the total number of candles presented, but the bright- 

 ness is the luminous intensity divided by the area of the source. 

 The brightness, then, is measured in terms of candles per unit 

 of area. In the light of this definition, Breed's use of the term 

 "brightness" (5) is also open to criticism. 



Regarding Johnson's assertion (15) that I reported "no con- 

 trol tests to show that discrimination was on the basis of size, 

 rather than of luminous intensity of the stimuli," it is sufficient 

 to note that variation of source distances would vary both 

 brightness and luminosity. Independent variation of these two 

 factors would be unnecessary so long as neither was allowed to 

 remain constant. This control is adequately described in my 

 report (2) on page 98. 



Johnson's method consists in presenting striate fields for dis- 

 crimination, adopting the visual angle subtended by one of the 

 striae as a convenient measure of the animal's visual acuity. 

 In terms of the visual angle subtended by individual striae, the 

 stimulus threshold for the chick was found to be slightly above 

 4'. The standard individual striae were about 0.11 mm. in 

 width, the assumption being that, at the given distance of 60 

 cm., "they were too small to be resolved by the eye." The 

 minimum size of the variable individual striae discriminated 

 from the standard striae was accepted for chick 1 as 0.710 mm. 

 and for chick 2 as 0.743 mm. (16). 



The refined dark room-color apparatus is used by Lashley (21) 

 in studying the spectrum of the chick. Preliminary to his study 

 of wave length and without using punishment as a motive, he 

 finds that the brightness threshold is approximately three to one. 

 In contrast with earlier work, this difference limen is unusually 

 large. Aside from certain trivial errors in computations, Lash- 

 ley's work seems to have been carefully done. He thinks that 

 field experimenters may feel confident that differential reactions 

 of birds to colored objects are made on the basis of wave length 

 if the objects do not differ enormously in brightness for the 

 experimenter. He concludes that "the chick can distinguish 

 between monochromatic lights of any intensity between thresh- 

 old and the Pfund standard, irrespective of the brightness or 

 saturation. The effective stimulus is the wave length." 



