If it is objected that Professor Hering’s theory as to continuity of vibrations being the key to memory and heredity involves the action of more wheels within wheels than our imagination can come near to comprehending, and also that it supposes this complexity of action as going on within a compass which no unaided eye can detect by reason of its littleness, so that we are carried into a fairy land with which sober people should have nothing to do, it may be answered that the case of light affords us an example of our being truly aware of a multitude of minute actions, the hundred million millionth part of which we should have declared to be beyond our ken, could we not incontestably prove that we notice and count them all with a very sufficient and creditable accuracy. “Who would not,” {59a} says Sir John Herschel, “ask for demonstration when told that a gnat’s wing, in its ordinary flight, beats many hundred times in a second? or that there exist animated and regularly organised beings many thousands of whose bodies laid close together would not extend to an inch? But what are these to the astonishing truths which modern optical inquiries have disclosed, which teach us that every point of a medium through which a ray of light passes is affected with a succession of periodical movements, recurring regularly at equal intervals, no less than five hundred millions of millions of times in a second; that it is by such movements communicated to the nerves of our eyes that we see; nay, more, that it is the _difference_ in the frequency of their recurrence which affects us with the sense of the diversity of colour; that, for instance, in acquiring the sensation of redness, our eyes are affected four hundred and eighty-two millions of millions of times; of yellowness, five hundred and forty-two millions of millions of times; and of violet, seven hundred and seven millions of millions of times per second? {59b} Do not such things sound more like the ravings of madmen than the sober conclusions of people in their waking senses? They are, nevertheless, conclusions to which any one may most certainly arrive who will only be at the pains of examining the chain of reasoning by which they have been obtained.”