Page:Popular Science Monthly Volume 28.djvu/881

Rh likely to be ague. The ancient inhabitants of the Campagna cleared it of disease by draining it so dry that the bacteria could not thrive in it. This ids considered impracticable at present, and our Roman investigators have turned their attention to the best prophylactics against malarial poison. The universal quinine is good, but there are objections to its constant use, and arsenic, cautiously administered, is suggested as preferable. Professor Tommasi-Crudelli recommends, as an alternative prophylactic, decoction of lemon. The plantation of the eucalyptus appears to have failed. Near Rome, at the "Tre Fontane," where eucalyptus-trees have been grown with a special view to settling the question of their virtue, both the monks who inhabit the monastery and the workmen whom they employ have suffered as much as others. In one summer, when the Campagna was comparatively free from malaria, the inhabitants and servants of the "Tre Fontane" suffered more than the rest. Year before last, Professor Tommasi-Crudelli advised the Italian Government to drain and cover with turf the grounds of the Palazzo Salviati on the Lungara, where the new military college has been built. This was done. The result was that no cases of malarial fever occurred, while on the other side of the road there were several that ended fatally.

Inertia of the Eye and the Brain.—In a paper on the "Inertia of the Eye and the Brain," Mr. James Mckeen Cattell, of the University of Leipsic, discusses, in view of the results of experiments which he has made, that part of the process of sensation which concerns the time a light must work on the retina in order that a sensation may be excited. The time is to a considerable extent dependent on the nature of the object and the intensity of the light. It varies with the several colors. Orange gives the quickest impression, and yellow is hardly behind it; next come blue, red. and preen; while the retina is least sensitive to violet light, the time for which is from two to three times as long as for orange. When lamp-light is substituted for daylight, the time required for perceiving the colors becomes longer, and the order is changed to orange, red, yellow, violet, and blue. When the intensity of colored light varies, the time increases in arithmetical progression us the intensity decreases in geometrical progression. Applied to the distinction of words and letters, the experiments showed that Roman letters are more quickly perceived than German letters, and that the time is slightly shorter for words than for letters, but longer for long or rare words, and for words in a foreign language. The simplest geometrical forms of the letters seem the easiest to see; all ornaments on the letters hinder; and it is doubtful whether it is advantageous to use the thin lines or two varieties of letters in priming. Our punctuation-marks are hard to see, and Mr. Cattell, believing them to be useless, suggests that they might be replaced by spaces between the words proportionate to the importance of the pause. Some of the letters, as S and C, are hard to recognize in themselves; others, as O, Q, G, and C, are liable to confusion by their similarity of form; while E is "needlessly illegible." The order of distinctness for the small letters is d, k, m, g, h, b, p, w, u, l, j, t, v, z, r, o, f, n, a, x, y, e, i, g, c, s. The letters are slightly more difficult to grasp than the numbers, for every combination of numbers makes a number that gives "sense." Not as many words as letters can be grasped at one time, but three times as many letters, when they make words, and twice as many words when they make a sentence, as when they have no connection. The sentence is taken up as a whole; if it is not grasped, hardly any of the words are read; if it is grasped, the words appear very distinct; and this is also the case when the observer constructs an imaginary sentence from the traces he has taken up. The personal equations were important factors in all the experiments, but they did not materially affect the results as wholes.

The Problem of London Sewage.—The disposition of the sewage of London has been made the subject of the report of a royal commission, but still remains nearly as dark as ever. The one point on which all are agreed is that the present method of turning the sewage and rainfall of the streets into the river near the city is reprehensible from every point of view, but it is almost