Page:Popular Science Monthly Volume 47.djvu/732

716 effective, a filtering apparatus must either remove or destroy any micro-organisms contained in the water.

Color Photography.—At a recent soirée of the Royal Society, in London, Dr. Joly, of Dublin, exhibited some photographic transparencies upon glass plates representing various objects in their natural colors. The subjects photographed were especially chosen because of variety of color and delicate shading, and were reproduced with great naturalness and fidelity. The results were accomplished by the use of a finely ruled glass plate, two hundred to three hundred lines to the inch, each three lines being a complete color series, consisting of an orange-yellow line, a greenish-yellow line, and a blue-violet line, these colors being repeated over and over again. The lines are ruled with colored inks, made up of gum and gelatin mixed in certain proportions, on a gelatin-coated plate. The plate to be exposed is placed in contact with this color-screen, and only exposed to light which has passed through the latter; an extra-long exposure is necessary, owing to the partial opacity of the color-screen. The plate is then developed in the ordinary way. The color-screen is now again placed against the negative, and when the two are held up to the light, if the color-screen is placed just as it was when the exposure took place, an accurately colored reproduction of the original scene appears. The process is so simple and inexpensive that it will probably come rapidly into general use.

The Value of Object Lessons.—In a recent educational circular we find the following on object teaching: "To sum up the main value of object teaching, there are three principal uses: The first and most important is to teach the children to observe, compare, and contrast; the second is to impart information; and the third is to re-enforce the other two by making the results of them the basis for instruction in language, drawing, number, modeling, and other handiwork. There are, however, other important uses of good object teaching. It makes the lives of the children more happy and interesting by opening up an easily accessible and attractive field for the exercise of brain, hand, and eye; it gives the children an opportunity of learning the simplest natural facts; and directs their attention to external objects, making their education less bookish. It further develops a love of Mature and an interest in living things, and corrects the tendency, which exists in many children, to destructiveness and thoughtless unkindness to animals, and shows the ignorance and cruelty of such conduct. The value of the services which many animals render to man should be dwelt upon, and the importance of kindly treating them and preserving them should be pointed out. By these means, and in other ways, good object teaching may lay the foundation for the right direction of the activity and intelligence of the children throughout the whole school."

Sunlight and Pictures.—The question of preventing or mitigating the fading of pictures and pigments has been attacked in earnest and in a practical way by Captain W. de W. Abney, who finds that fading in the course of time is one of the inevitable effects of the operation of ordinary sunlight. Pictures can not well be taken from the light, so the next best thing is to discover which of the solar rays do the most damage, and to mitigate their effects as far as possible. The violet rays prove to be most active in producing fading. If we can eliminate the majority of these rays from white light without appreciably altering the freshness of the colors viewed in such light, we shall practically have prolonged the life of a picture. A variety of experiments made with different pigments tell us that the loss of the violet of the spectrum is practically no loss at all. Even with white light the loss is unnoticeable. If we form a patch of light composed of all the colors except the violet, we shall notice but little change from the pure white that is alongside of it. The case becomes simpler yet when we find that the blue-green light and the yellow light of the spectrum superposed give substantially white. A blue-green glass and a yellow glass interposed against the sunlight practically cut off all the violet, while they give passage to the rays that form white. Captain Abney therefore solves his problem by using glasses of these colors for the window-glazing of his gallery. Making the windows