280 Animal Life and Intelligence. 



quently they are in abnormal proportions. They probably 

 vary in their sensitiveness, and not improbably in the 

 wave-period to which they show the maximum response. 



To test the variation, if any, in the limits of instability 

 for E. and V., or in any case in the limits of colour- vision 

 at the red end and at the violet end of the spectrum, in 

 apparently normal individuals, my friend and colleague, 

 Mr. A. P. Chattock, made, at my suggestion, a number of 

 observations on some of the students of the University 

 College, Bristol, to whom my best thanks are due for their 

 kind willingness to be submitted to experiment. The instru- 

 ment used * was a single-prism spectre-goniometer. 



In the accompanying diagram (Fig. 33) the results of 

 some of these observations are graphically shown. The 

 middle part of the spectrum, between the wave-lengths 

 420 and 740 millionths of a millimetre, is omitted, only the 

 red end and the violet end being shown. The observations 

 on thirty-four individuals, seventeen men and seventeen 

 women, all under thirty years of age, are given for both 

 eyes. The left-hand vertical line of each pair stands for 

 the right eye in each case. To the left of the table are 

 placed the wave-lengths in millionths of a millimetre. 



Take, for example, the first pair of vertical lines. The 



* Mr. Chattock has kindly supplied me with the following note: 



" Headings at the violet end were taken at the extremity of the lavender 

 rays, at the point where the faint band of lavender light seemed to end off 

 about half-way across the field of view (the cross-wires being invisible). 



" At the red end the cross-wires were always visible, and were in each 

 case set to the point where the top horizontal edge of the spectrum lost its 

 definition. 



" Other things equal, the ' red ' readings should be more reliable than the 

 violet, therefore, from the greater definiteness of the point observed, and the 

 means of observing it. But against this has to be set off the fact that the 

 extreme violet rays were spread out by the prism used more than eight times 

 as much as the red rays. 



" In any case, the wide differences observed in the ' red ' readings are much 

 greater than could have been due to misunderstanding or careless observation 

 as shown by setting the instrument to maximum and minimum readings, 

 and noting the very obvious difference between them apparent to a normal 

 eye. The same conclusion is rather borne out by the closer (average) agree- 

 ment between the two eyes of the same individual than between those of 

 different persons. 



" The source of light was the central portion of an ordinary Argand burner." 



