still smaller image, scarcely visible, in fact, with the one twen ty-fifth screen in which the proportion of light and darkness is well marked, being better marked with the finer screens. In the intermediate distances we have upon our screen no image, but simply a luminous spot. If we repeat this experiment with white light, we find the circular net acting like a non-achromatic and very dispersive lens. At the distance proper for the red rays, the image is red, surrounded by a blue aureole. On removing the screen further, the image changes through the yellow and the green, and finally becomes blue; having in the latter case a red aureole. From this latter experiment Soret shows that we can consider the little circular screens as concave lenses, so that the ordinary Galilean telescope may be constructed by employing such a screen instead of the eye-glass. Bull. Hebdom., XVI., 71. THE COLOR OF DIAMONDS. Flight and Maskelyne have lately made some curious observations upon colored diamonds. It has for some time been known that the tints of these stones are either destroyed or modified by heating, the change being sometimes temporary, sometimes permanent. In the present case two yellowish diamonds from the Cape of Good Hope were strongly heated in an atmosphere of hydrogen in a porcelain tube, for about two hours. Upon cooling, the color of the stones was found to have vanished, but it returned after exposure, for only a few minutes, to diffused light. In one instance a diamond which had been decolorized by heat was kept in the dark for three days, and remained colorless; but an exposure of six or seven minutes to the light again brought back its yellowish hue. These facts appear to stand in some relation to phenomena of phosphorescence.-15 C, XXIX., 33. GILT GLASS PRISM IN THE CONSTRUCTION OF THE CAMERA LUCIDA. By taking advantage of the property possessed by thin metallic films of allowing the passage of direct rays through them, while they reflect oblique rays from some other source, Professor Govi, of Rome, has devised a perfect method for superposing a direct and reflected image, as is necessary the camera-lucida, without the usual fatigue to the eye. He in simply gilds the reflecting surface of the prism, in the camera-lucida, with a very thin film of gold, and, with Canada balsam, cements to this surface another similar prism. The reflected image appears of the usual yellow color of gold by reflected light, while the transmitted one is green, a difference that is not only not annoying, but in some cases serviceable. The suggestion of Professor Govi has been adopted by Nachet in the construction of various forms of camera-lucida.—14 C, CCXIII., 1874, 447. COMPRESSIBILITY OF WATER. Mascart has shown that the compressibility of water varies more rapidly than the pressure, as has already been observed for other liquids. The methods used by him to investigate the subject have also induced him to measure the heat evolved by the compression of water; or, rather, the lowering of temperature when the pressure is suddenly removed.—4 D,VII., 593. NEW PHOTOMETERS. Major Elliot reports that in his visit to the establishment of Messrs. Chance, at Birmingham, the scientific director in charge, Dr. Hopkinson, presented him with a photometer of his own invention for the comparison of lights at a distance. It is very compact, and consists of two Nicol prisms which can be moved relatively to each other in azimuth. A little tube carries the analyzing prism, and a second tube contains the polarizing prism. The latter being turned until the observed light is just diminished to the point of invisibility, and another light being then observed in the same way, a comparison of the angles gives the relations of the powers of the lights. The French Light-house Commission employ a photometer different from Bunsen's, as ordinarily used in America, in which, instead of keeping both the standard light and the one under test fixed in position, the former is moved until the beams from both, after passing through a slit or opening in the photometer, fall upon a pane of glass which has a ground surface, and which, as viewed on the reverse side, seems equally illuminated by the two lights. The distances from the photometer are then measured by a tape line, and reference to a calculated table shows at once the G intensity of the light under test, in terms of the standard, or unit, which in France is always the Carcel burner, consuming forty grammes of colza oil. This French unit is estimated to be equal to between nine and a half and eleven and a half of the English units or candles.-Elliot's European Lighthouse System, p. 183. THE REFLECTION OF LIGHT. An almost exhaustive historical essay, by Lundquist, on the investigations of earlier physicists into the peculiarities of the light reflected from the surfaces of solid bodies, is supplemented by observations made by himself on the reflection. from fuchsin and some other substances. The methods followed by him were similar to those adopted of late years by Jamin, Wiedemann, Van der Willigen, and others. A narrow pencil of sunlight, reflected in a fixed horizontal direction from a heliostat, passes successively through an achromatic lens, a flint-glass prism, and a polarizing Nicol's prism, and falls upon the reflecting surface of fuchsin; the reflected light is then analyzed by a compensator and a second Nicol's prism. Rays of light from seven different portions of the spectrum were examined; and Lundquist concludes that, in respect to the principal angle of incidence, fuchsin comports itself as does indigo, and the observations are represented by the theoretical formulæ for metallic reflection so long as the angle of incidence is greater than 50°. The author's investigation into the intensity of the reflected light shows that, on the one hand, the intensity is always slightly less than that computed, and that, on the other hand, the quantities reflected vary sensibly with the color of the incident light, so that when white light falls upon the fuchsin the color of the reflected rays varies with the angle of incidence. The power of the substance to absorb different colored rays offers a remarkable anomaly, as, while the yellow light is reflected in greater proportion than the blue, it is absorbed in less proportion.-Poggendorff Annalen, CLII., 595. THE ACTION OF LIGHT UPON CHLOROPHYLL. It has been long known that alcoholic extracts of chlorophyll are decomposed rapidly in the sunlight, but slowly in diffused daylight, and in even the faintest light assume vari ous colors. The presence of oxygen is necessary, according to Wiesner, in order that the rays of light may effect the decomposition of chlorophyll, and this author has made an interesting investigation into the action of different portions of the spectrum. He concludes that all the chemical changes caused in chlorophyll cells by the rays of light (namely, their development, decomposition, and assimilation of other substances) take place most rapidly in the brightest portion of the spectrum; and that, while all portions of the visible spectrum have the power of inducing these changes, the mechanical effects of light upon plants are to be especially ascribed to the rays of high refrangibility.- Poggendorff Annalen, CLII., 503. EXPERIMENTS ON THE VELOCITY OF LIGHT. While astronomy has been busy with the problem of the sun's distance, physics has contributed an independent solution to this question. M. Cornu, of the French Academy of Sciences, has repeated the celebrated experiments on the velocity of light which were proposed by Foucault and Fizeau some twenty-five years ago. He has himself improved the method of Fizeau, and his experiments were exhaustively conducted and have been perfectly successful. M. Cornu chose for his two stations the Observatory of Paris and the tower of Montlhéry, whose distance apart (about fifteen miles) is very precisely known. The beam of light passing from the observatory fell upon a toothed wheel revolving no less than 1600 times per second; a portion of the beam, escaping through the interval between two teeth, passed on to a reflector at Montlhéry, and returned thence to the revolving wheel. If the rotation of the wheel during the time required for the light to travel to and fro over twice the distance between the two stations interposes a tooth in the path of the returning ray, an extinction of the luminous impression occurs, and from the known velocity of rotation of the wheel, and from the known distance of the stations, the velocity of light can be had. By revolving the wheel at different rates this extinction can be made to occur at the fourth, fifth, sixth, etc., tooth. A great accordance characterizes Cornu's results, and high importance attaches to this delicate research. All the observations were made at night by the aid of a Drummond light, except one which was AUTOMATIC REGISTRATION OF THE CHEMICAL ACTION OF LIGHT. The measurement of the chemical intensity of the solar light has not yet become a subject of regular meteorological observation, because of the want of a proper instrument. This want is now partly supplied by a method proposed by Roscoe, who proposes to effect the measurement by the blackening of a paper saturated with chloride of silver; or, rather, by means of the time required in order that the exposure to the light may bring about a given intensity of shade. A uniformly prepared paper is placed in the apparatus during the previous night, and is, by a mechanical arrangement, hourly exposed during a given interval to the sunlight. In order to estimate correctly the intensity of the solar action, Roscoe arranges the apparatus so that the paper shall, each hour, many times in quick succession, be exposed to the light for from two to thirty seconds. We thus have, at each hour of the day, a complete series of small spots of various tints, and have only to seek that tint which corresponds to the normal to know at once the number of seconds of exposure corresponding to the strength of the sunlight at that time. A series of observations made during the months of May, |