Page:Popular Science Monthly Volume 15.djvu/153

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been reached. But, if the intensity of the stimulus be now as gradually diminished, the eye will continue to perceive it till it has fallen to one third or one fourth of the original minimum. In producing the initial sensation a certain amount of light has, so to speak, been wasted in putting the machinery in motion. Further, if the eye has been carefully shielded from the light for some minutes before performing the experiment, it will be capable of perceiving light which is fifty or even one hundred times less intense than that required to produce a luminous sensation. This enormous difference is equally manifested whether monochromatic or white light be employed. Now, if we apply a similar test to the sensation of color, we find that for the chromatic as for the luminous stimulus a certain minimum is needed to produce the sensation, which still continues to be excited when the intensity of the stimulus is progressively diminished. So far, the two sensations, of light and color, obey the same law. But if we proceed to compare the sensitiveness of the eye in full activity with that of the eye which has been allowed a period of absolute rest, we no longer find any such increase in its susceptibility to the chromatic stimulus as was observed in the case of light. This result is altogether opposed to the current opinion that the sensation excited by white light is really a resultant of the simultaneous development of several determinate color sensations; it shows, on the contrary, that the sensation of light is altogether independent of that of color, and really a simpler kind of reaction on the part of the visual apparatus.

"Oil on the Troubled Waters."—The fishermen of the Shetland Isles, as we learn from a writer in "Chambers's Journal," are wont, when in utmost peril during a storm, to throw oil on the waters to still them. They crush in their hands the livers of any ling or cod they may have caught, and keep throwing them astern and around them. "The effect," we are told, "is magical. The waves are not lessened in size; but they no longer break, and it is only from their breaking close to the boat and so being dashed in upon her and filling her that there is danger. The rapidity with which the oil spreads over a considerable space of sea around is marvelous, and scarcely to be credited except by one who has witnessed the phenomenon." An expedient so simple might often be of invaluable service in saving life and property. The difficulty and peril, for instance, of launching a boat from a sinking ship in a storm are mostly caused by the wind breaking the waves over the boat and filling her or dashing her against the vessel's side. "The danger of such a mishap would unquestionably be greatly lessened by throwing overboard some oil, which ought always to be kept handy. Boats also going from one ship to the assistance of another in distress, and life-boats on their way to a wreck, and boarding it, might often with very great advantage use a little oil, if its effects were only better known. Another case in which oil might be of the greatest service is when a man accidentally falls or is washed over-board. Life-buoys are thrown into the sea, the ship is brought to as quickly as possible, boats are lowered and a search made; but, before all this can be done, the vessel has run a considerable distance, and, although the poor struggler in the water may be a good swimmer and able to keep afloat for some time, the great difficulty is to find the exact spot where he is to be sought for. A life-buoy or a man's head is a small object to descry among heaving waves and white foam. If life-buoys were constructed so as to contain a small portion of oil in a little receptacle or India-rubber bag attached to them, to be punctured with a knife before being thrown overboard, the effect would be not only to prevent the sea from breaking over the castaway, so making it easier for him to keep afloat, but would indicate to the searchers almost the exact spot where to look for him."


The commonly received theory of dew is that it results from the condensation of the moisture of the air by contact with surfaces of a lower temperature. This theory is rejected by Professor Stockbridge, of the Massachusetts Agricultural College. He holds dew to be the vapor from the soil condensed by the cooler air, and states as follows the results of his experiments: 1. The vapor of the soil is much warmer at night than the air, and would be condensed by it. 2. Va-