in these spectra interference bands, which disappear in any given part of the field common to the two spectra when the intensities are equal. The instrument is an ordinary spectroscope, the two spectra being obtained, the one above the other, by the common device of a reflection prism. Between, now, the collimator and the prism the author places a Foucault prism to polarize the beam horizontally, a plate of quartz cut parallel to the axis to produce a considerable difference of path between the ordinary and extraordinary rays, and a Wollaston prism to bring back the light rays into two planes at right angles to each other, and at the same time to double the images of each half of the slit. Four channelled spectra are now visible in the instrument, two of these (one polarized vertically, the other horizontally) overlapping. Consequently the brilliant maxima of the one coincide with the dark maxima of the other; and when the intensities are equal, the bands completely disappear. The method is simple and satisfactory.

Rücker has given, in Nature, an account of some interesting experiments with black soap films, i. e., films of soap and water so thin that no light is reflected by them, and they appear black. He has observed that, under whatever conditions the black film may have been formed, a remarkable and very rapid change of thickness invariably occurs at the boundary which divides the black from the colored portion of the film. By an exceedingly happy method, the thickness of the film was measured by measuring the resistance of a known area. The value of the resistance of a black ring one millimeter broad was 1,750,000 ohms, from which the calculated thickness is twelve millionths of a millimeter, or one forty-ninth part of the wave-length of D. Various measurements prove this thickness to be approximately uniform.

Thompson has communicated to the London Physical Society, says Nature, a paper on interference fringes within the Nicol prism. If the "field" of a Nicol be explored by the eye, it will be seen to be bordered on one side by a margin of violet-blue light, and on the other, when the light passes obliquely through the prism, by an orange band, within which lie a series of colored fringes; these latter are very clearly seen with monochromatic light, when a second set within the blue band also appears. The author showed that

these two sets are due to interference taking place within the film of balsam at the critical angle of total reflection for ordinary and extraordinary rays respectively; they are, therefore, analogous to the interference bands in a thin film placed beneath a prism of a more highly refracting substance, and occurring just within the limit of total internal reflection, as first observed by Sir W. Herschel.

Stone has exhibited to the London Physical Society some diffraction gratings on glass and metal, ruled by W. Clark, of Windsor Terrace. The majority of them were close spirals, about 1000 to the inch, which gave brilliant circular spectra, the slight difference between spirals and true circles not being apparent. The metal gratings were linear, 1000 to the inch, the spectra being much more brilliant than the refracted ones. The German silver and cast steel hitherto employed not being suitable, the author proposes the use of speculum metal. The idea is not new, Saxton having ruled lines for this purpose for Bache many years ago. The exquisite speculum-metal gratings of Rutherford are well known. He much prefers them to glass silvered, and his latest triumphs in this direction abundantly justify the preference.

André has studied theoretically the phenomena of diffraction in optical instruments and their influence on astronomical observations.

Bezold has suggested another and a very convenient method for studying the laws of color-mixture. A prism of Iceland spar is placed in the interior of a blackened tube, which is closed below by a disk having four squares cut out of it. The prism, of course, gives, on looking through it, double images of the squares, and in a certain position two of the eight are brought to coincide with two others in the middle. Surfaces of different colors being brought under the two squares occupying, say, the upper row, their composite. color is obtained in the middle image. It is then easy to find what color must be put under the lower two squares to obtain a color in the middle corresponding to the one above.

Baily has examined microscopically the optical properties of starch grains. He concludes that they are transparent bodies, consisting of an interior nucleus surrounded by coats, and explains their appearance in polarized light by supposing the starch to be doubly refracting, with two axes of elas

ticity at each point in the plane of the disk, one of which is directed towards the centre of this disk.

Soret and Sarasin have investigated the rotatory power of quartz, extending their observations to the ultra-violet rays as far as the line R, using in general the method of Foucault and Fizeau, modified by that of Mascart. The numbers obtained agree well with those calculated by Boltzmann's formula.

Henri Becquerel has submitted a large number of substances to the influence of magnetism, and examined them with polarized light, to determine whether the magnetic rotatory polarization of these bodies sustained any relation to their other physical properties. He finds that this magnetic rotation in general increases with the refractive index of the body, nearly as n2 (n2—1), and draws a number of important facts from this conclusion.

ELECTRICITY.

1. Magnetism.

Wild has examined the properties of a nickel-magnet made by Joseph Wharton, of Philadelphia, and given by him to Kotschubey, of the Russian commission. The magnet was 155 millimeters long, 9.5 wide, and 2 thick, the ends being pointed. It weighed 25 grams. Its magnetic moment, determined in the usual way, was, per gram, 112,000 as received, and 186,000 after remagnetizing; while that of a nearly similar steel magnet was 245,000 and 368,000 respectively. On analysis by Butlerow, the only impurity was iron, of which there was present one third of one per cent.; traces of cobalt were also detected. Wild concludes, 1st, pure nickel, unlike iron, takes considerable permanent magnetism, but the amount is only from one half to one third of that taken by different sorts of hardened steel; 2d, the magnetism remaining in nickel is less permanent than in hardened steel; 3d, the temperature-coefficient is less in the case of nickel than in that of hardened steel; and, 4th, the temporary magnetism acquirable by nickel, though about twice its permanent magnetism, is only half that which hardened steel and only one fourth of that which soft iron is capable of acquiring.

Du Moncel has studied the relation which should exist between the diameter of iron cores and the thickness of their magnetizing helix, and finds from experiment that there is an advantage in winding electro-magnets so that the thickness of the coil-layers is equal to the diameter of the cores. Moreover, the diameter of the cores should naturally be proportioned to the electric intensity which is to act on them, and be so chosen that they shall be nearly saturated by the

current.

Helmholtz has communicated to the Academy of Berlin a paper containing the results of experiments by Rowland, which satisfactorily prove that electric convection currents are dynamically equivalent to the flow of electricity in a conductor, and are electro-magnetically operative.

2. Electromotors.

Zöllner has investigated a new class of electrical phenomena hitherto but imperfectly known. When two different bodies, an insulator and a half-conducting rubbing instrument, are rubbed together, electrical currents occur in the rubber as follows: if the rubbed insulator be positively electric, the currents at the surface of contact or in the interior of the rubber are parallel, but opposite to the relative motion of the insulator; if the latter be negative, the currents of the rubber are parallel and in the same direction as the insulator's motion. These currents were measured, and shown to be often very considerable. They could be intensified by multiplying the rubbers and connecting their corresponding parts with wires. They lessen the useful effect of an electrical machine, in which, however, a positive advantage is had by uniting the electricity of the positive end of the rubber with the positive electricity of the conductor. The author has also studied a variety of related experiments, as, for example, the currents generated by the flow of water through a thin tube. Zöllner concludes that diaphragm currents and their modifications are due to the occurrence of new electro-motive forces, such that the electric current they generate in the moved liquid, so long as it is in contact with the canals of the diaphragm or the capillary tube, are always opposite to an electric current which would force the liquid in the same direction through the diaphragm

F

which is caused by the mechanical pressure which is oper

ative.

Fuller has communicated to the Physical Society of London a description of a new form of electrical machine, called an electric multiplier, which is essentially a double-acting electrophorus. It consists of a vulcanite quadrant, over which electricity is distributed by a metallic arm, carrying needles which swing over it, the charge being removed from the other side by a similar set of points attached to an arm connected to earth. Two quadrantal metal plates being now automatically brought in contact with the vulcanite, receive a strong charge, which becomes free on removing again the plates from the vulcanite.

Hagenbach has made some experiments in the university of Basle to ascertain the effective performance of one of the smaller Gramme magneto-electric machines used as a source of light for lecture-room purposes. He finds as follows:

The work done at 1800 revolutions was 90 kilogrammeters, or about 475 candles to the horse-power. The machine was driven by a water motor, and the light cost nearly seventy cents per hour. As to give the same light from a battery would require from seventy-two to eighty Bunsen cells, the light from the machine is much the cheapest.

Anthony has also given some measurements of the performance of a Gramme machine constructed in the workshop of the Cornell University. According to the results given, the machine, when making but about 500 rotations, gave a light of 1600 candles; but as, unfortunately, the horse-power consumed is not stated, no opinion of the efficiency of the machine can be formed. Further experiments are promised.

3. Velocity.

A paper has been published by Wand on the propagation of electricity in cylindrical conductors, which is an extended mathematical discussion of the whole subject. He finds (1)