It is the usual method of testing for colour blindness, in which, from defects of vision, certain colours can not be perceived at all.

It answers very well for colour blindness; for what we really want to learn in the case of a sailor or a signal-man is whether he can recognise a given signal when it is repeated; that is, does he know green or red to be the same as his former experience of green or red?
But it is evident that there is still a more fundamental question in the matter--the real question of colour perception.

It is quite possible that a child might not recognise an isolated colour when he could really very well distinguish the colours lying side by side.

The last question, then, is this: When does the child get the different colour _Sensations_ (not recognitions), and in what order?
To solve this question it would seem that experiments should be made upon younger children.

The results described above were all secured after the children had made considerable progress in learning to speak.
To meet this requirement another method may be used which can be applied to children less than a year old.