376 Mr. H. E. Ives on a 



keep pace ivitli the appreciation of brightness difference. How- 

 necessary this eight- to ten-fold difference in the relative hue 

 and brightness discriminations for steady and flicker condi- 

 tions is, is shown graphically in fig. 7, where the upper 

 dashed colour flicker curve (calculated by the aid of equa- 

 tion (11)), is the one which would condition the sensibility if 

 the two discrimination fractions maintained the same relative 

 values they have for steady juxtaposed fields. There would 

 be no sensibility at all. This was further verified by at- 

 tempting to operate the mixture photometer with the two- 

 glasses turned to the point of just noticeable hue difference. 

 Either light source could then be placed practically any- 

 where on the photometer track without altering the bright- 

 ness match. 



This breakdown of hue discrimination is, of course, only 

 another way of stating the common " explanation " of the 

 flicker photometer, that " colour flicker ceases before bright- 

 ness flicker." This is, however, the first time that definite 

 measurements have been made of the visual constants whose 

 values result in this condition. The explanation now needed 

 is one step further back, namely, why does hue difference 

 discrimination fail in the ultimate receiving apparatus with 

 rapidly alternating impressions ? 



It may be suggested, as a start toward an explanation, that 

 colour discrimination is probably a later development than 

 brightness discrimination, and that under conditions of con- 

 fusion or stress the more primitive function maintains its 

 characteristics with less impairment. The supposition may 

 be hazarded that in general with the senses appreciation of 

 quality differences is much easier disturbed than that of in- 

 tensity differences. A close parallel to the action of the 

 visual apparatus, as revealed by these measurements, is 

 furnished by a flow-meter, such as a gas-meter, to which 

 is attached an apparatus for indicating the kind of gas going 

 through, the latter being conceived as a more complicated 

 device requiring some time to respond. As long as only one 

 kind of gas flows through the measurement of rate and the 

 indication of composition are equally satisfactory. But let 

 two different gases flow through in rapidly varying propor- 

 tions. The precision of the gas-meter will be unchanged, 

 but the composition indicator will be quite unable to respond 

 to each momentary condition. 



