Some time ago Mr. J.D. Hardy devised an instrument, which he has named a chromatoscope, so easily made by any one who has a spot lens that we take the following description from the _Journal_ of the Royal Microscopical Society: "Its chief purpose is that of illuminating and defining objects which are nonpolarizable, in a similar manner to that in which the polariscope defines polarizable objects. It can also be applied to many polarizable objects. This quality, combined with the transmission of a greater amount of light than is obtainable by the polariscope, renders objects thus seen much more effective. It is constructed as follows: Into the tube of the spot lens a short tube is made to move freely and easily. This inner tube has a double flange, the outer one, which is milled, for rotating, and the inner one for carrying a glass plate. This plate is made of flat, clear glass, and upon it are cemented by a very small quantity of balsam three pieces of colored (stained) glass, blue, red, and green, in the proportion of about 8, 5, and 3. The light from the lamp is allowed to pass to some extent through the interspaces, and is by comparison a strong yellow, thus giving four principal colors. Secondary colors are formed by a combination of the rays in passing through the spot lens. "The stained glass should be as rich in color and as good in quality as possible, and a better effect is obtained by three pieces of stained glass than by a number of small pieces. The application of the chromatoscope is almost unlimited, as it can be used with all objectives up to the 1/8. Transparent objects, particularly crystals which will not polarize, diatoms, infusoria, palates of mollusks, etc., can not only be seen to greater advantage, but their parts can be more easily studied. As its cost is merely nominal, it can be applied to every instrument, large or small; and when its merits and its utility by practice are known, I am confident that it will be considered a valuable accessory to the microscope." * * * * * Prof. W.O. Atwater, as the results of a series of experiments, finds, contrary to the general opinion of chemists, that plants assimilate nitrogen from the atmosphere. They take up the greatest quantity when supplied with abundant nourishment from the soil. Well fed plants acquired fully one-half their total nitrogen from the air. It seems probable that the free nitrogen of the air is in some way assimilated by the plants. * * * * * A catalogue, containing brief notices of many important scientific papers heretofore published in the SUPPLEMENT, may be had gratis at this office. * * * * * THE SCIENTIFIC AMERICAN SUPPLEMENT. PUBLISHED WEEKLY. TERMS OF SUBSCRIPTION, $5 A YEAR. Sent by mail, postage prepaid, to subscribers in any part of the United States or Canada. Six dollars a year, sent, prepaid, to any foreign country.