Furthermore, no single stimulus, as a colour, should be twice repeated without a change to some other, since the child's eagerness or alertness is somewhat satisfied by the first effort, and a new thing is necessary to bring him out to full exercise again.

After each effort or two the child should be given the object reached for to hold or play with for a moment; otherwise he grows to apprehend that the whole affair is a case of "Tantalus." In all these matters very much depends upon the knowledge and care of the experimenter, and his ability to keep the child in a normal condition of pleasurable muscular exercise throughout.
In performing colour experiments, several requirements would appear to be necessary for exact results.

Should not the colours chosen be equal in purity, intensity, lustre, illumination, etc.?
In reference to these differences, I think only that degree of care need be exercised which good comparative judgment provides.

Colours of about equal objective intensity, of no gloss, of relatively evident spectral purity, under constant illumination--this is all that is required.

The variations due to the grosser factors I have mentioned--such as condition of attention, physical unrest, disturbing noises, sights, etc .-- are of greater influence than any of these more recondite variations in the stimulus.