Page:Popular Science Monthly Volume 15.djvu/586

 conditions of the present time on the character of those which preceded and gave rise to them. Of the many classes of problems falling under these heads two are specified: The one deals with the reciprocal influence of man and his surroundings, showing on the one hand the influence of external nature on race, commercial development and sociology, and on the other the influence of man on nature, in the clearing of forests, cultivation and drainage of the soil, introduction of new plants and domestic animals, and the like. The other problem deals with the inferences that may be drawn from the present distribution of plants and animals in respect to the configuration of the surface of the earth in ancient times. Thus we see that the mutual relations of the different sciences is the subject of a science in itself, so that scientific geography may be defined as the study of local correlations.

What the Eyes see in reading.—On page 838 of our fourteenth volume we published some remarks by M. Javel on the impairment of eyesight caused by habitual protracted reading. M. Javel has since published some further observations on the mode in which the eye "takes in" the successive letters on a printed page. We are not to suppose, he says, that in reading a line one passes successively from the lower part of a letter to the upper part, then down the next letter, up the next, and so on, the vision describing a wavy line. The fixation takes place with extreme precision along a straight line, traversing the junction of the upper third of the letter with the lower two thirds. Why is this line not in the middle? Because characteristic parts of the letters are more frequently above than below, in the proportion of about seventy-five per cent. That this is so, we can see by applying on a line of typographic characters a sheet of paper covering the line in its lower two thirds, and leaving the upper third exposed. We can then read the letters almost as well as if they had not been concealed in greater part. But the case is very different if we cover the upper two thirds of the line; the lowest third alone does not furnish sufficient for recognition. The characteristic part of the letters, then, is chiefly in their upper portion. M. Javel next compares the ancient typographic characters with those of modern books, and maintains that the latter have too much uniformity, so that, taken in their upper parts alone, many of them may be confounded in reading. The old letters, on the other hand, had each a particular sign by which they could be easily distinguished. The Elzevirian a, for example, has no resemblance to o, the r could not be confounded with the n, as now, nor the c or e with o, the b with h, etc. This too great uniformity in the upper part of typographic characters should be corrected, since it is to that part we chiefly look in reading.

Professor Vaughan on the Origin of the Asteroids.—It is to the general features of the numerous small planets beyond Mars that we must look for a record of their past history. Though most liable to elude the search of the observers, asteroids of great orbital inclination to the ecliptic show already such numbers as to prove a stumbling-block to the usual methods of inquiry respecting the primitive condition of the planetary group. If, as Laplace supposes, they are parts of a ruptured solar ring, they could never deviate far from the plane of Jupiter's orbit by the attractive influence of other planets, and the greatness of the deviation as well as its independence of size excludes the idea of its arising from any mutual action exerted among themselves. The hypothesis of Olbers fails also to meet the difficulty. In the explosion of a single planet moving in the plane of the ecliptic, such an impulse as produced the great inclinations of the paths of Pallas, Euphrosyne, Gallia, Electra, or Artemis, would give cometary orbits to masses launched forth in opposite tangential directions, and cause many asteroids of such an origin to wander far beyond the zone between Mars and Jupiter. Yet the peculiarities in question are precisely such as may be expected in a group of bodies owing their birth to a collision of two planets not very unequal in size or mass. Such an event would be possible when by long disturbance the orbits of both become too eccentric for safety, and when the aphelion of one was in conjunction with the perihelion of the other and near the intersection of their planes. Supposing both