[v] The advantages and defects of the optical structure of our human eyes have been carefully estimated by Helmholtz. He has also discussed the difficulties attending eyesight considered as a sensation and perception. Extracts from his clear yet popular Lectures are given in Additional Note B.
[93] Proceedings of the Royal Institution. V. 456.
[94] All theories of light require these immense numbers. Sir J. Herschel says there is no "mode of conceiving the subject which does not call upon us to admit the exertion of mechanical forces which may well be termed infinite." The numeration in the text is a rough and ready shape of statement at once intelligible. But it is interesting to view the subject more exactly.—Light travels in one second 192,000 miles. Each mile contains 63,360 inches, and in each inch are 39,000 waves of red light, calculated at their mean length. Now, multiply these three sets of figures together, and we get a rate of 474,439,680,000,000 red waves per second. The mean length of a violet wave is the 1/57500th part of an inch; and by a like multiplication we find a product of 699,494,400,000,000 of violet light-strokes thrown upon the retina in each second. The phrase "millions of millions" is used in the text, because few people realize the idea of any arithmetical whole beyond a million.
[95] "What we hear" writes Professor Max Müller "when listening to a chorus or a symphony is a commotion of elastic air, of which the wildest sea would give a very inadequate image. The lowest tone which the ear perceives is due to about 30 vibrations in one second, the highest to about 4,000. Consider then what happens in a Presto when thousands of voices and instruments are simultaneously producing waves of air, each wave crossing the other, not only like the surface waves of the water, but like spherical bodies, and, as it would seem, without any perceptible disturbance; consider that each tone is accompanied by secondary tones, that each instrument has its peculiar timbre, due to secondary vibrations; and, lastly, let us remember that all this cross-fire of waves, all this whirlpool of sound, is moderated by laws which determine what we call harmony, and by certain traditions or habits which determine what we call melody—both these elements being absent in the songs of birds—that all this must be reflected like a microscopic photograph on the two small organs of hearing, and there excite not only perception, but perception followed by a new feeling even more mysterious, which we call either pleasure or pain; and it will be clear that we are surrounded on all sides by miracles transcending all we are accustomed to call miraculous, and yet disclosing to the genius of an Euler or a Newton laws which admit of the most minute mathematical determination." Science of Language, Second Series, p. 115.
[96] There is a much more scientific mode of trying this experiment. A description of the instrument (Kaleidophone), and cuts of the figures produced, may be seen in Tyndall on Sound, pp. 132. seq.
[97] There is reason for believing that a large proportion of animal eyes see much as ours do when in a normal state. Colour blindness is frequent in Man and occurs between red and green, yet a bull distinguishes the two like a healthy, human being. He is allured by the sight of a green field, and lashes himself into fury when a red rag is waved before him.
The eyes of insects are very far removed in structure from ours. A butterfly's compound eye contains 17,000 tubes, that of the Mordella beetle 25,000. Their perception of colours appears vivid and distinct. They resemble birds, reptiles, and other creatures in choosing for their lairs and resting-places objects coloured like themselves. It is not difficult to mount one of these compound eyes, so as to look through it by aid of a lens placed in focus. Leeuwenhoeck looked through the eye of a dragon fly (made up of 12,544 tubes), "and viewed the steeple of a church which was 299 feet high, and 750 feet from the place where he stood. He could plainly see the steeple, though not apparently larger than the point of a fine needle. He also viewed a house in the same manner, and could discern the front, distinguish the doors and windows, and perceive whether they were open or shut." See Insect Miscellanies, p. 129.
[98] Two points connected with colour admit of being easily experimented on, and deserve from their interest to be made the subjects of repeated observations.
The first has relation to the question of primary colours;—are they alike in man and in all the lower animals?—In birds and reptiles there are anatomical reasons for believing the primaries to be red, yellow, and blue. But are they the same in our race?—may they not more probably be red, green, and violet? In this case yellow is the transition from red to green, blue from green to violet. As colour blindness consists in an insensibility to red, and as the outer circle of the field of vision is feeble in its reds, the number of experiments which might be suggested is evidently considerable.
Let a person place two threads respectively red and green near the bridge of the nose, so as to be seen by the inner angle of the pupil only. If dexterously moved, both seem green;—if not, both will in time become black. Where the want of sensitive appreciation of red is great, the same result follows in every part of the field of sight. Thus reverend gentlemen in former times have been induced to wear scarlet hose under the impression that they had put on black silk; and in these railroad days many persons find themselves unable to distinguish between the safety and the danger signal lights. It seems strange indeed that any scientific advisers of railway Boards should have recommended for use the two colours, above all others, most likely to get confounded.