COMMUNICATION WITH THE PLANETS. 363 



eighty-five miles. Tliis 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 sup- 

 poses they are, would have, in case they should desire to start an 

 exchange of telegraphic communications with their earthly neigh- 

 bors, 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 distin- 

 guish nothing. The earth will be better situated at the quadra- 

 tures, 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 con- 

 vinced 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. 



The compilation of a digest of the literature of the mathematical sciences was 

 sugi?ested at the American Association by Prof. Alexander S. Christie. Tne 

 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 througliout the world should be invited to 

 engage in the preparation of the work, and the co-operation of the British Asso- 

 ciation especially should be secured. 



There 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 movemepts of 

 plants are largely determined by the relations of liglit. 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, Engel- 

 mann, and Loeb in Protozoa ; and other authors have observed in Bryozoa^ coelen- 

 terates, Spongiaria^ worms, larvas of arthropods, and isolated organs of mollusks 



