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TESTING MICROSCOPE OBJECT-GLASSES.

Dr. Pigott advocates the method of testing the objectglasses of microscopes by examination of the miniatures reflected from small globules, especially the examination of the sun's image as seen reflected in small globules of mercury. In this method, an object-glass of fine quality is screwed into the sub-stage of the microscope in an inverted position. On black velvet there are scattered, from a syringe containing mercury, a number of mercurial globules; then, by means of a prism, a brilliant light is thrown vertically downward upon them. The object-glass to be tested is now screwed to the nose of the microscope. The two objectives are brought to a central position, so that their axes coincide, and the instrument is then adjusted to form. miniatures of the globules for examination. The most beautiful effects are seen under sunlight. The miniatures develop appearances of marvelous beauty and variety. The aperture of the miniature-making objective should be at least as wide as the objective to be tested, and the lens of the finest quality obtainable. Among the innumerable illuminated objects that may be used, Dr. Pigott strongly recommends what he calls the fundamental experiment; that is, a disk of intense light as small as possible, viewed from a distance sufficiently great to develop the test dif fraction rings. It is well known that the surface of the illuminated globules of mercury becomes more nearly spherical as they diminish in weight. The law of the curvature of these surfaces dependent upon the specific attraction of mercury has been investigated by Professor Bashforth, though not yet published. Under direct illumination, a minute spectrum of the sun may be described. The symmetry, beauty, and fineness of refraction rings exhibited by these miniatures from illuminated globules of mercury are severe tests of the objective, and afford delicate means of adjusting its corrections.- Monthly Microscopic Journal, 1875, 147.

ACCIDENTAL OR SUBJECTIVE COLORS.

Mr. Plateau states that observations made by himself upon a number of persons of his own acquaintance have

shown him that it is impossible to adopt the principle that
the accidental or subjective color observed when we cease
contemplating a bright object has always a tint comple-
mentary to that of the object itself. This subjective tint
depends upon the eyes of the observer; and the cases where
the principle is satisfactory constitute rather the exception
than the rule, at least so far as concerns blue and yellow.
Therefore, in a recent communication to the Belgium Acad-
emy, he says that he must continue to maintain the general
theory, with reference to accidental colors, that he published
forty years ago, and which he thinks has not been sufficient-
ly considered by recent authors. His theory consists essen-
tially in the following propositions: First, during the con-
templation of a colored object, the retina exerts an increas
ing reaction against the action of the light which falls upon
it, and tends to throw itself into an opposite state. Conse-
quently, after the disappearance of the object, it takes spon-
taneously, or, as it were, by inertia, its opposite state, whence
results the sensation of the accidental or subjective color.
Then it comes to repose by a sort of oscillation, in virtue of
which it passes alternately from the accidental to the com-
plementary tint, and vice versa. The physiological condi-
tion of the retina after the prolonged action of light is very
nearly like the state of a body which, drawn from a position
of stable equilibrium, then abandoned to itself, returns to
repose by a series of decreasing oscillations. Second, anal-
ogous phenomena take place in reference to space; while
one portion of the retina is submitted to the action of a col-
ored light, the surrounding portions throw themselves into
the opposite state; whence results, all around the colored
image, an aureole of the accidental colors, and, finally, be-
yond this aureole there is a tendency toward a manifesta-
tion of a cloudiness of the same tint as that of the primitive
image. Such a state of the retina can but be compared to
that of a vibrating surface, in which the nodal lines are sep-
arated by vibrations in opposite directions. This theory
has, he says, been adopted in France, but is quite rejected
by the German and English physicists. He has been so oc-
cupied by his extensive researches into the phenomena of
thin liquid films that he has not until lately found time to
defend his theory. In the memoir in question he adduces

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numerous observations tending to disprove the theories of Scherffer, and of those who have followed him, as also the theories of Thomas Young and his followers.-Bull. Ac. Roy., Belgique, 1875, 100.

REFLECTION OF THIN FILMS.

Govi has made a happy application of that principle in optics by which thin films can at the same time reflect and transmit rays of light according to their angles of incidence., He applies a film of gold to the hypothenuse of a right-angled prism of glass; the film allows direct rays to pass through the prism, while the latter reflects the oblique rays coming through the microscope. By placing this prism obliquely upon the ocular of a microscope, the magnified image is reflected upon a sheet of paper, where it can be drawn by the observer who looks through the gold film.— Rev. Sci., 1874, 167.

SIEMENS'S ELECTRICAL PYROMETER, AND DIFFERENTIAL VOL

TAMETER.

We had occasion some years ago to give some account of the very elaborate investigations made by Weinhold in reference to the reliability of the various methods employed for measuring very high temperatures accurately, and to call attention to the fact that his researches fully substantiated the claim of Dr. Siemens that the electrical pyrometer, as constructed by him, was a thoroughly reliable instrument. It is now a pleasure to be able to refer to the very impor tant memoir of Dr. Siemens himself, just published in the Journal of the Society of Telegraph Engineers. This memoir, which was in part delivered as a lecture in 1871, has been delayed in its publication, owing to the innumerable interruptions experienced by the author in consequence of his professional duties. The fullness of its details shows how large a series of experiments Dr. Siemens undertook to satisfy himself of the accuracy of his method of measuring temperatures.

His memoir consists, first of all, of a very suggestive chapter on the influence of temperature upon the electrical resist. ance of metallic conductors, which he expresses as a function not only of the temperature reckoned from the absolute zero,

but, first, of the co-efficient of increase peculiar to the particular metal under consideration; second, of the co-efficient of increase dependent upon the co-efficient of the expansion of the metal; and, third, of a function of a co-efficient expressing the resistance of the material at the absolute zero. He finds his formula correctly applicable to the metals platinum, iron, copper, aluminum, and silver, at temperatures between zero and 350° C. The results of his experiments are given in detail, and afford a valuable basis for still further investigations.

In the second part of his memoir he states that, in 1860, when engaged in examining the electrical condition of the Malta and Alexandria telegraph cable, his attention was directed toward the fact that the increase in the temperature of the cable could be measured by the increasing resistance to the electrical current; and accordingly constructed coils of cable wire, of known electrical resistance, inclosed hermetically in iron tubes, out of which passed thick insulated wires; and placing these coils at various points within the mass of the cable, he was able, by examining the varying electrical resistances, to ascertain that the interior of the large mass of coiled cable was steadily rising in tempera ture, and by pouring cold water thereon saved it from ultimate destruction. Following up this idea, he shortly afterward constructed thermometer coils, consisting of a spiral or insulated wire, inclosed in a cylindrical silver casing, which he used for measuring ordinary temperatures on land, and which could be used, he suggested, by physiologists and others. The instrument is extremely sensitive, being correct within one tenth of a degree Fahr., or even less; and a modified arrangement of this kind for measuring deep-sea temperatures was presented to the Berlin Academy in 1863. After describing the method adopted by him for determining the temperature of a distant spot, and also a similar apparatus furnished by him to the steamship Challenger in her exploring expedition, he gives in detail the method of construction of the pyrometer for measuring high temperatures. He states that Professor Bolzain, of Kasan, is at present em ploying his resistance thermometer for registering the temperatures below the surface of the earth, and measuring the temperature of the air above; and, furthermore, that Mr. Bell,

the eminent metallurgist, habitually employs his pyrometer for the determination of the temperatures employed in various operations of the blast furnace.

The third part of Mr. Siemens's paper is a highly suggestive and valuable memoir on a simple method of measuring electrical resistances. He states that although a Wheatstone balance furnishes electricians with the means of measuring the resistances of electrical circuits with great accuracy, yet its application is, in many cases, rendered difficult on account of the delicacy of the apparatus and of extraneous disturbing causes. As a portable instrument, and one especially applicable to observations on shipboard and in exploring expeditions, he proposes what he calls a differential voltameter, which consists of two similar narrow closed tubes fixed vertically to a wooden frame, with a divided scale behind them, and whose lower ends, being enlarged somewhat, are fitted with wooden stoppers saturated with paraffin, and penetrated by platinum wires. Diluted sulphuric acid is admitted into these tubes, and kept at a proper height in each by a very simple device, and the evolution of gas that occurs, when a current passes from the electrodes, affords the measure of the strength of the current. By means of a commutator the current from the battery is easily reversed every few seconds, preventing polarization of the electrodes. By introducing the resistance of the voltameter, and the unknown resistance x, first on one, and then on the other side of the arrangement, the observations, by a simple arithmetical process, give the exact value of the unknown resistance. The measurement of the quantity of decomposed gases serves merely to determine the relative intensity of the currents which flow in the respective positions of the commutator. He states that, having measured numerous resistances by this instrument, and compared the results with measurements obtained by a very perfect Wheatstone bridge arrangement, he finds that it may be relied upon within a half per cent. of error of observation. The instrument especially recommends itself for use on board of vessels, as not being in the slightest degree influenced either by the motion of the vessel or by the magnetic influence of moving masses of iron. One of its intrinsical advantages is that it gives the resistance to be measured in units of work done,

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