a telescope. He introduces between the ordinary eye-piece and the eye of the observer a small tube containing a cylindrical convex lens of about four inches' focal length, for which lens, under different circumstances, we may substitute other lenses of different lengths of focus. Within the tube containing this lens, and between it and the eye, there is inserted a second tube holding an ordinary direct-vision prism, such as is made by Browning. The intensity of light in this ocular spectroscope is so considerable that in combination with a small portable telescope of one and a half inches' aperture it shows distinctly the lines of stars of the first magnitude, and of the crescent of Venus. It is peculiarly applicable to the systematic observation of star spectra in which the main object is to ascertain the typical constitution of the spectra.— 7 A, XLVIII., 156.

THE BEGINNINGS OF SPECTRUM ANALYSIS.

According to Lockyer, the distinguished Swedish philosopher Angström, whose untimely death in June, 1874, was at the time chronicled by us, will be forever considered as the founder of spectrum analysis, although unfortunately the obstacles opposed by the language in which his first treatise was written, and by distance from the scenes of his investigations, for three years prevented even its existence from being known to the scientific world at large. His work, "Optiska Undersokningar," published at Stockholm in 1853, was the first publication in which use was made of a principle already propounded by Euler, viz., that the particles of a body in consequence of resonance absorb principally those æthereal undulating motions which are impressed upon them. He also endeavored to show that a body heated until it glows emits the same kind of light and heat which it absorbs under other circumstances; he further stated that in many cases the Fraunhofer lines are the inversion of the bright lines which are observed in the spectra of various metals in the galvanic arch.-12 A, X., 377.

SPECTRA OF CERTAIN RARER METALS.

Professor Thalen has published the result of an investiga tion into the spectra of the rarer metals yttrium, erbium, didymium, and lanthanium. He has operated with large

quantities of the metals of undoubted purity, and his results are the most reliable hitherto obtained. He has not only removed all doubt with regard to some 28 spectral lines that were hitherto known, but has increased the whole number of these lines peculiar to these metals from 160 to 590.

EFFECT OF TEMPERATURE AND PRESSURE ON THE SPECTRUM LINES.

The question has often been discussed whether it is temperature or pressure which causes the widening of the lines in the spectrum of any gas. The following considerations. are adduced by Schuster as favoring the view that each separate molecule would show at all temperatures narrow lines only, but that the shocks of the other molecules cause the widening, which may therefore be considered as depending rather upon pressure than temperature. Frankland and Lockyer have found that if we increase the pressure of hydrogen while the electric current is passing through it, the lines begin to expand until the spectrum becomes continuous, and, finally, the current ceases to pass altogether. On the other hand, Gassiot has observed that if we diminish the pressure of hydrogen, its electric resistance diminishes, becomes a minimum, and then increases again. We are therefore compelled to accept Frankland and Lockyer's original conclusion that pressure and not heat is the cause of the widening of the line.-Rep. Brit. Assoc., 1873, 39.

NEW TABLES OF SPECTRUM LINES.

The committee appointed by the British Association to construct a catalogue of spectral rays state in their recent report that the whole of their work is now finished and ready for the printer, so far as regards the solar spectrum, while the positions of the metallic lines, as determined by Thalen, have been only partly reduced to uniformity with the rest of the work. The tables presented by them are constructed by throwing the solar lines into those groups which catch the eye in observing the spectrum, and the position of each line has been corrected for the dispersion of air. Both Kirchhoff's and Angström's scales will be given with the adopted wave-lengths for each spectral line, so that it is hoped that, when these catalogues are printed, observers will find in them,

in a collected form, the best materials which yet exist for the identification of lines, and for the reduction of fresh determinations to wave-lengths.-Rep. Brit. Assoc., 1873, 250.

ADVANTAGEOUS CONSTRUCTION OF THE SPECTROSCOPE.

In some remarks upon the optics of the spectroscope, Mr. Sorby states that in the construction of a spectroscope the eye-piece should be of as long a focus as possible, so as to cause all the rays to enter the eye. All magnifying beyond this means loss of brilliancy; and if the spectrum appears insufficiently large, an increase in the size of the collimating and telescope lenses, together with the prisms, or an increase in the number of prisms, should be made, until the spectrum appears large enough to suit the requirements of the observer.-12 A, X., 469.

ABBE'S REFRACTOMETER.

The new instrument devised lately by Professor E. Abbe, of Jena, for the determination of the index of refraction of any transparent body, has received high encomiums, and promises to be of use in all optical researches. In principle it is, we understand, based upon the property of total reflection of light. Two similar right-angled prisms are so fixed that their hypothenuses are parallel and a slight distance apart. Between them is placed the liquid to be examined, and by measuring the angle through which this combination must be turned in order to secure the total reflection of a ray of light, we have the means of directly determining the index of refraction. In order to employ white light, and annul the indistinctness caused by the dispersion of light, a compensator is introduced, based upon a combination of flint and ground-glass prisms, and it is stated by Professor Waltenhofer that the execution of measurements with this instrument leaves nothing to be desired as regards rapidity, elegance, and accuracy.-Technische Blätter, 1874, 106.

THE CAUSE OF THE LUMINOSITY AND NON-LUMINOSITY OF

FLAMES.

It has been ascertained that if nitrogen, hydrochloric acid, or carbon dioxide be passed into the flame of a Bunsen burner, it becomes non-luminous; but when any such mixture

becomes strongly heated before it undergoes combustion it again becomes luminous.

From these observations it may be concluded that the non-luminosity of a gas flame is not caused by a dilution of the gas, this dilution being in fact increased by heating the mixture. The only cause is the cooling of the interior of the flame. This is further proved by the fact that it is most difficult to get a non-luminous flame from a mixture of coal-gas and oxygen, showing that neither rapid oxidation nor dilution produces the non-luminosity. — 21 A, July, 603.

FLAME OF BURNING GLYCERINE.

According to Godeffroy, glycerine burns with a steady blue non-luminous flame, without diffusing any odor or leaving any residue.-17 A, June 1, 84.

A PERFECTLY MONOCHROMATIC SODIUM FLAME.

Laurent recommends the following simple method for rendering the light proceeding from a soda flame, which it is desired to use for saccharometric and similar work, perfectly monochromatic. Between the flame and the polarizer he interposes a thin lamina of potassium bichromate, which possesses the property of absorbing the violet, blue, and green rays contained in the sodium flame, the presence of which impairs the accuracy of observations involving a comparison or determination of the equality of tints.-2 C, III., 1875, 62.

AN APPARATUS FOR ILLUSTRATING THE MECHANICAL EFFECTS OF LIGHT.

Dr. William Crookes is the first to illustrate by experiment the production of direct mechanical effects by the action of luminous rays. The apparatus, with the aid of which the demonstration was made, and to which its designer gives the name of the radiometer, is described as follows: The radiometer consists of four small pith disks, fixed at the extremities of two crossed arms of straw, balanced on a pivot at the point where the straws intersect, so that they may freely spin round. These pith disks are white on one side and blackened with lampblack on the other, and the entire arrangement is inclosed in a glass bulb from which the air has

been removed with the help of a Sprengel pump. With an apparatus of this description, the disks and arms spun around rapidly when luminous rays were directed upon it, but obscure heat rays produced no effect upon it. When submitted to the action of light from which 95 per cent. of the heating rays had been cut out by the interposition of a plate of alum, the disks still revolved, though with somewhat diminished velocity. Contrary to expectation, it was the blackened faces of the disks that were repelled by the light.

THE SPECTROSCOPE WITH A FLUORESCENT OCULAR,

In observing the most refrangible portions of the spectrum it is common to use two methods-either the spectrum falls upon a plate sensitive to the ultra violet rays, or else the spectrum falls upon a fluorescent substance which has the property of revealing these rays. To this a third method is now added by Soret, whose application is very simple. Soret's method consists in this, that he inserts a plate of transparent fluorescent substance in the tube of the spectroscope, and observes the spectrum with a telescope whose ocular is inclined to its axis. With a plate of uranium glass the fluorescent spectrum is well seen. The best substance, however, is a somewhat concentrated solution of aesculin, by means of which the spectrum may be traced up to the line O. This simple apparatus affords an especially convenient method for examining the spectrum of the ultra violet porion of the solar light.-19 C, VII., 232.

ON THE INTENSITY OF THE LIGHT REFLECTED FROM GLASS.

Dr. Glau states that hitherto the investigations of the properties of reflected light have, by preference, referred to the ratio of the two principal components to each other, as well in respect to their phases as to their intensity; and in but few cases has it been attempted to make a direct comparison of these two components in regard to the incident light. The only experiments on the intensity are found in Arago's works, and are quite fragmentary, and on that account the inquiry has been undertaken anew by Dr. Glau, in order especially to test the accuracy of Cauchy's formula. His observations give the ratio of the intensity of the