Special experiments seem to show that their limits of vision at the red end of the spectrum coincide approximately with ours; but at the violet end their spectrum is longer than ours. Sir John covered up the visible spectrum, so as to render it dark, and gave the daphnias the option of collecting in this dark space or in the ultra-violet. To human eyes both were alike dark. But not so to the daphnian eye; for while only 14 collected in the covered part, 286 were found in the ultra-violet. The width of the violet visible to man was two inches. Sir John divided the ultra-violet into three spaces of two inches each. Of the 286 daphnias, 261 were in the space nearest the violet, 25 in the next space, and none in the furthest of the three spaces. From which it would seem that, though these little creatures are sensitive to light of higher vibration-period than that which affects the human eye, their limits do not very far exceed ours. We have seen that human beings differ not a little in their limits of violet-susceptibility. We may presume that Sir John Lubbock and those who assisted him in these experiments were normal in this respect. But it is possible that some individuals could have perceived a faint purple where there was darkness to them, and that the majority of the 261 daphnias were collected in the region just beyond the partition between ultra-violet and darkened violet. Still, there is no cause for doubting the general conclusion that daphnias are sensible to ultra-violet rays beyond the limits of human vision.

Fig. 40.—Antennary structures of hymenoptera. (After Lubbock.)

a., cuticle; b., hypodermis; c., ordinary hair; d., tactile hair; e., cone; f., depressed hair lying over g. cup with rudimentary hair at the base; h., simple cup; i., champagne-cork-like organ of Forel; k., flask-like organ; l., papilla, with a rudimentary hair at the apex.

Sir John Lubbock has an interesting chapter on problematical organs of sense. In the antennæ of ants and bees there are modified hairs and pits in the integument (at least eight different types, according to Sir John Lubbock), the sensory nature of which is undoubted. But what the sensory nature in each case may be is more or less problematical. Many worms have sense-hairs or bristles of the use of which we are ignorant. Some organs described as tactile or olfactory in the lower invertebrates are so described on a somewhat slender basis of evidence. The sense-value of the bright marginal beads of sea-anemones is unknown. Even in animals as high in the scale of life as fishes, there is a complete set of sense-organs—the muciparous canals, in the head and along the lateral line down the side, the function of which we can only guess. By some they are regarded as olfactory; by others, as fitted to respond to vibrations or shocks of greater wave-length than the auditory organ can appreciate; by others, as of importance for the equilibration or balancing of the fish.

It will thus be seen that, apart from the possibility of unknown receptive organs as completely hidden from anatomical and microscopic scrutiny as the end-organs of our temperature-sense, there are in the lower animals organs which may be fitted to receive modes of influence to which we human folk are not attuned.

And what are the physical possibilities? We have seen that, through the telæsthetic senses—hearing, vision, and the temperature-sense—we are made aware of the vibrations of distant bodies, the effects of which are borne to us on waves of air or of æther. The limits of hearing with us are between thirty and about forty thousand (or perhaps, in very rare cases, fifty thousand) vibrations per second. But these are by no means the limits of vibrations of the same class. By experiments with sensitive flames,[FN] Lord Rayleigh has detected vibrations of fifty-six thousand per second; and Mr. W. F. Barrett has shown that a sensitive flame two feet long is sensitive to vibrations beyond the limit of his own hearing and that of several of his friends who were present at the experiment. We have some reason to suppose that vibrations too rapid to be audible by man are audible by insects, but not much is known with regard to the exact limits.

The following table shows what is known concerning the æther-vibrations. The figures are those given by Professor Langley:—

Quality of radiations.Wave-lengths in
thousandths of
a millimetre.		Number of
vibrations per second
in billions.		Effects on man.
Limit of photography, artificial source0.185160none known
Limit of photography,solar source0.295 none known
Limit of violet to normal eyes0.36833}vision.
Limit of red to normal eyes0.81370
Probable inferior limit of temperature-sensations9.25[FO]30temperature-sense
Longest waves hitherto recognised with bolometer30.01none known