Page:Popular Science Monthly Volume 40.djvu/377

Rh eighty-five miles. This minimum can, it is true, be reduced by using large objectives permitting stronger magnifying; but even then it is certain that luminous signals, for example, visible from the earth on Mars, must have enormous dimensions.

The inhabitants of Mars, if more advanced in astronomical knowledge than we, as one of our imaginative astronomers supposes they are, would have, in case they should desire to start an exchange of telegraphic communications with their earthly neighbors, to give their signals diameters of miles in every direction. But would they think of it? The reciprocal question to this is the one that puzzles me. The earth, during all the oppositions of Mars, is in conjunction to it. It is lost in the rays of the sun, and invisible from Mars, unless it is in transit over the sun's disk. Then it is a little black, round spot, on which we have every reason to suppose the Martian astronomers will be able to distinguish nothing. The earth will be better situated at the quadratures, but also at a much greater distance.

I stop here, not desiring to discourage absolutely the candidate for the prize of one hundred thousand francs so generously and so imprudently offered to investigators. But my conclusion, which I have sufficiently foreshadowed, is, that the problem of interplanetary communication is still far from solution; and I believe I shall never be contradicted by real astronomers. I have faith in the indefinite progress of the science, while I am convinced that there are limits to this progress; but I believe also that there is no profit in letting the imagination chase chimeras, and I am free to avow that the desired communication is such to my eyes.—Translated for The Popular Science Monthly from La Nature.

compilation of a digest of the literature of the mathematical sciences was suggested at the American Association by Prof. Alexander S. Christie. The digest should contain everything of value hitherto done in these sciences logically arranged, with each truth or method referred to its discoverer, and the whole thoroughly indexed. Mathematicians throughout the world should be invited to engage in the preparation of the work, and the co-operation of the British Association especially should be secured.

is no doubt that a kind of perception of light exists even among beings that have no visual organs, or where such organs can not be brought into play. The property is perhaps not unlike that by which the growth and movements of plants are largely determined by the relations of light. A number of cases of such skin perceptions of light—which we might call dermatoptic or photodermatic—have been collected and described by M. Victor Willem in a French journal. Tremblay observed that hydras prefer the more illuminated parts of the medium in which they move; and the same has been remarked by Haeckel, Pouchet, Engelmann, and Loeb in Protozoa; and other authors have observed in Bryozoa coelenterates, Spongiaria, worms, larvas of arthropods, and isolated organs of mollusks