On Water and A ir. 
1 27 
1880.] 
if I can ; but before I do that I will tell you the meaning of 
what I have said about the persistence of impressions upon 
the retina. Every boy has burnt a stick and twirled it 
round so as to form a circle of fire in the air. The appear- 
ance of that circle is due to the impression on the retina. 
If you complete the circle in the fifth or the eighth of a 
second— for the time differs in different eyes— you see not 
simply the burnt end of the stick, but a circle of fire. 1 he 
impression produced upon the eye when the end of the stick 
begins the circle is not extinguished until the end comes 
round again to its starting point ; and thus you have a con- 
tinuous circle of light. We can in this way mix colours in 
the eye. You saw the spedtrum on the last occasion, and I 
told you then that if we were to mix all these colours 
together we should reproduce the white light from which 
that spedtrum came. Newton’s way of doing it was to 
colour a disc with what he called the primitive colours, and 
Fig. 10. 
D 
then to impart the motion of rotation to the disc. If the 
motion be quick enough to enable the eye to retain the 
impression of all the colours during a revolution of the disc, 
then all the colours will be shown, as it were, simultaneously 
to the eye, and the colour will disappear, and the disc will 
be rendered white. This disc will now be turned, and you 
will see immediately that the colours will disappear. [A 
disc of card containing the prismatic colours in due propor- 
tions was caused to rotatewith great rapidity so that the indi- 
vidual colours could no longer be distinguished, and the disc 
appeared white.] It is an experiment which I dare say most 
of you understand perfectly. Now, in place of the colour disc 
we will use a pasteboard circle with black and white seg- 
ments. We might do this in a more perfect form with a 
larger apparatus, but I want to do it in a simple manner. 
