For that was just what those “eyes” did. In the nearly perfect vacuum of this little world’s surface, gases diffused at high speed — and their molecules traveled in practically straight lines. There was nothing wrong with the idea of a pinhole camera eye, whose retina was composed of olfactory nerve endings rather than the rods and cones of photosensitive organs.

That seemed to account for everything. Of course the creatures were indifferent to the amount of light reflected from the object they examined. The glare of the open spaces under Deneb’s rays, and the relative blackness of a cave, were all one to them — provided something were diffusing molecules in the neighborhood. And what doesn’t? Every substance, solid or liquid, has its vapor pressure; under Deneb’s rays even some rather unlikely materials probably vaporized enough to affect the organs of these life forms —

metals, particularly. The life fluid of the creatures was obviously metal — probably lead, tin, bismuth, or some similar metals, or still more probably, several of them in a mixture that carried the substances vital to the life of their body cells. Probably much of the makeup of those cells was in the form of colloidal metals.

But that was the business of the biochemists. Cunningham amused himself for a time by imagining the analogy between smell and color which must exist here; light gases, such as oxygen and nitrogen, must be rare, and the tiny quantities that leaked from his suit would be absolutely new to the creatures that intercepted them. He must have affected their nervous systems the way fire did those of terrestrial wild animals. No wonder even the centipede had thought discretion the better part of valor!