Wave-lengths Number of in thousandths vibrations Quality of radiations. of a per second Effects on man. millimetre. in billions. Limit of photography, artificial source 0.185 160 none known Limit of photography, solar source 0.295 none known Limit of violet to normal eyes 0.36 833 } Limit of red to normal eyes 0.81 370 } vision. Probable inferior limit of temperature-sensations 9.25[FO] 30 temperature-sense Longest waves hitherto recognised with bolometer 30.0 1 none known From this table it will be seen that, apart from the possible extension of sight beyond human limits, there are possibilities of another sense for the ultra-violet actinic vibrations as different from sight as is the infra-red temperature-sense. Moreover, the temperature-sense for us has no scale; there is nothing corresponding to pitch in sound or colour in sight. It may not be so with lower organisms. Insects, for example, may be sensitive to tones of heat. The bee may enjoy a symphony of solar radiance. I am not saying that it is so; I am merely suggesting possibilities which we have not sufficient knowledge to authoritatively deny. We have no right to impose the limits of human sensation on the entire organic world. Insects may have "permanent possibilities of sensation" denied to us. Even within our limits there may be, as we have already seen, great and inconceivable differences. We saw that our own colour-sensations are probably due to the blending and overlapping in different proportions of three primitive monochromatic bands, but that in all probability in birds the bands are different, and overlapping is largely prevented. Their colour-phenomena must be inconceivably different from ours. And what shall we say of the colour-vision of invertebrates? Are we justified in supposing that for them, as for us, R., G., and V. are the unstable explosives, and that they are present in the same proportions as with us? If not, their colour-world cannot be the same as ours. Of the same order it probably is. And all that we can hope to do is to show, as has been shown, that colours which differently affect us affect them also differently. * * * * * In conclusion, we may return to the point from which we set out. The organism is fitted to respond to certain influences of the external world. The organs for the reception of these influences are the sense-organs. When they are stimulated waves of change are transmitted inwards to the great nerve-centres; they are there co-ordinated, and issue thence to muscles or glands. Thus the organism is fitted to respond to the influences from without. The activities of organisms are in response to stimulation.