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view objects at different distances, but there is a limit to the distance of distinct vision. It varies from five to ten inches in different persons. An object cannot be distinctly seen nearer than either one or other of these distances, or some intermediate point. Now optical science supplies a means of overcoming this difficulty. An object apparently enlarged by being brought nearer to the eye, becomes indistinct, because the pencils of rays proceeding from it diverge too much to be brought to a focus on the retina by the lens of the eye.

The optical instrument most closely resembling the eye is the camera obscura. When the rack is run out to its full extent and the body is drawn out as far as it will go, and the object is still too near to be focussed distinctly, the position is precisely that of the eye with an object too close to it for distinct vision.

The remedy with the camera, supposing that no more focussing space could be obtained, would be to shorten the focus of the lens; that is, use one of greater refractive power. The same thing must be done in the case of indistinct vision, by interposing a convex lens and so increasing the convergent power of the lens of the eye; the object, hitherto too near the eye, a, b, is thus rendered visible and clearly defined, but with this remarkable difference, that it appears not to be the image of the object placed near the eye, but of a larger object, A, B, at the distance of distinct vision. No crossing of the rays takes place, and consequently the image is not inverted.

FIG. 6.*

or simple microscopes. The older observers had no other instruments. These may consist of many lenses, while the compound microscope may be made with as few as two; but then they act upon the principle that no image is formed, but the object is rendered visible at a shorter distance than it could be without optical aid. Single microscopes are still in use for dissection and other purposes where only a low power is required. And various contrivances have been adopted by which both eyes may be used and much fatigue saved in protracted operations.

The compound microscope differs from the simple instrument just mentioned in an image being formed and again magnified by being viewed with another lens or eye-piece (Fig. 7).

orl

The hand magnifier is an extremely useful instrument, but many persons find great trouble in using it to advantage, especially when it is of short focus and high power. A trifling hint about its use may be of value.

Hold the object in the left hand, either in the fingers or with forceps, and, placing the left hand against the right which is holding the lens, look through the magnifier and adjust the focus. The view is usually most distinct when the eye is in one focus of the lens and the object in the other; the two hands being held in contact move together and prevent that tremor which would be apparent, especially with high powers, if the hands were kept apart, and which unsteadiness prevents any accurate view of the object being obtained. Single lenses, when their diameter exceeds that of the pupil, possess the property of bringing light to the eye and rendering the object clearer than it would be without such assistance. This may be proved by looking at print or other suitable object in a bad light and noticing how much the magnifier aids by its light-collecting power.

Single lenses are mounted on stands of various kinds, and then they become what are called single Figs. 6, 7, 8, and 10 are taken from "The Microscope and its Revelations," by W. B. Carpenter, M.D. (Churchill).

From what has been said before respecting the formation of images, it is evident if a lens of short focus is used and an object placed before it, the image will be formed at a great distance behind it and be much enlarged; if instead of viewing this image directly, as in the camera obscura, it is viewed with a suitable lens, another enlargement takes place. Such an instrument might be constructed with only two lenses, as in the diagram Fig. 7, A, but a microscope so made would of course be a very imperfect instrument; indeed it would in almost every point be inferior in its performance to that of a simple lens, principally because the eye lens magnifies all the

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imperfections of the image formed by the small lens or object-glass. These imperfections are chiefly caused by the spherical and chromatic aberrations of the object-glass, and also in some degree by those of the eye-piece.

Theoretically, convex lenses bring parallel rays to a point called the focus, at a distance from the lens dependent upon the radii of their curved sides; practically, this is not the case; the rays passing through the marginal portion of the lens come to a focus at a shorter distance from it than those which pass through or near its centre (Fig. 8). Those

FIG. 8.

passing through intermediate portions converge to some part between these two points; the result is that indistinct images are formed at various points between F and f, Fig. 8, no well-defined figure being seen anywhere. Could lenses be made having an hyperbolic or elliptic section, the spherical aberration would be corrected, but the difficulties of making lenses of such figures are so great as to be considered impossible in practice. Spherical aberration may be lessened by reducing the aperture of the lens, by cutting off the marginal portion by means of stops or diaphragms; but any great use of this means causes a loss of light very detrimental to the performance of the instrument. Advantage may also be taken of the position in which the lens is placed and also of its figure; the spherical aberration of some lenses, such as plano-convex and meniscus, is much affected by the way they are placed, being much greater in one position than other. Much may be done by placing the glasses in their most favourable position and using suitable curves: as a striking example, take to pieces a photographic lens and observe the curious forms adopted in its construction. The details of these corrections are rather a matter for the practical optician, and are unsuitable for an elementary treatise on the microscope.

FIG. 9.*

Chromatic aberration is caused by the unequal refraction of the constituents of a ray of light. If a ray be bent by passing through a glass prism, instead of the ray appearing on the other side as white light it will be decomposed, owing to the unequal capability of bending which the rays possess; the red ray is bent *Hardwich's "Photographic Chemistry," p. 173 (Churchill).

least, the yellow more, and the blue and violet most of all (Fig 9). This also takes place when light is caused to pass through a lens (Fig. 10). The blue ray FIG. 10.

being the most refrangible comes to a focus nearest to the lens at b; the red ray being the least refrangible will not converge until it reaches r; the yellow, green, &c., rays come to a focus at intermediate points, so that a series of images surrounded by fringes of their respective colours would be formed at various points between b and r, such coloured margins rendering the definition extremely defective. Chromatic aberration may be lessened by the use of stops, cutting off the marginal portion of the lens, but at the sacrifice of a great quantity of light and the impairing of some of the most valuable qualities of the glass for microscopic purposes. Fortunately, a means has been found of almost entirely correcting chromatic aberration. Different kinds of glass vary not only in their refractive power but also in the degree in which they disperse the coloured rays, and, by combining lenses of suitable forms and materials, the error of one lens is so neutralised by the opposite error of the other that the combination as a whole is nearly free from the production of colour, or becomes, as it is termed, achromatic. For the practical application of this valuable fact, we are indebted to the elder Dollond, who constructed telescopes on this principle towards the end of the last century. The object glass of the microscope was not improved until many years afterwards.

Owing to the very divergent condition of the rays proceeding from a minute object placed near the lens of a microscope, the construction of an achromatic object-glass is much more complex than the combination employed in other optical instruments. High powers contain as many as three combinations of compound lenses placed behind each other, the careful adaptation of which requires the greatest skill on the part of the optician, and necessarily causes objectives of large aperture and great perfection to be very costly. A valuable series of papers by Mr. F. H. Wenham on the construction of object-glasses for microscopes will be found in the Monthly Microscopical Journal, vol. 1., pp. 111, &c.

So perfectly was the correction of the chromatic and spherical aberrations effected, that it was found that even the covering of the object to be viewed with a thin plate of glass or mica rendered the image sensibly indistinct: fortunately the defect was no sooner discovered than the late Andrew Ross applied a simple remedy; some alteration was made in the disposition of the correcting media, and the front combination allowed a small range of motion backwards and forwards, which is usually regulated by a graduated collar, the use of which it will be well to explain, as, although it is to be found in most works on the microscope, it seems to be generally overlooked. Makers differ a little in the details of their arrangement for this correction, but there will always be found some mark indicating when the object-glass is corrected, for viewing an uncovered object. Focus

siderable extent, the process of combustion; and it is supposed that the chemical rays act, in some way, on the portion of oxygen about to combine with the fuel, so as to delay, if not prevent, combination.

Supposing, in these experiments, the taper was so uniform, that one inch contained precisely the same quantity of matter as another inch, the time occupied in burning was too short to justify so important a conclusion as Dr.

McKeever arrived at, whether the results were taken by

measure or by weight.

Everyone engaged in photometrical observations must be aware of the difficulty of getting rid of disturbing causes and perplexing results. In comparing candles of the same make, the light is affected, both in quantity and economy, by a number of small circumstances, such as the warmth of the room, the existence of slight currents of air, the extent to which the wick curls over in burning, and so on. In testing the quality of gas, the standard candle defined by Act of Parliament is a sperm candle of six to the pound, burning at the rate of 120 grains per hour. From such a standard, we get the terms "twelve candle gas," "fourteen candle gas," &c. Mr. Sugg, in his "Gas Manipulation," has pointed out some of the difficulties in obtaining a uniform standard candle. The wick does not always contain the same number of strands ; they are not all twisted to the same degree of hardness; the so-called sperm may vary in composition--one candle containing a little more wax than another, or variable quantities of stearine or of paraffin; the candle may have been kept in store a long or a short time; the temperature of the store-room may have varied considerably, and the temperature of the room in which it was burnt may have been high or low. All these circumstances affect the rate of combustion, and the illuminating power of candles, irrespective of the action of light, if such action really exist.

I have lately had a good opportunity of testing this action at the works of Price's Patent Candle Company, at Battersea. Uuder the direction of Mr. Hatcher, the accomplished chemist of the Company, the greatest possible care is taken to ensure identity of composition and illuminating power in candles of the same name. There has lately been an extensive series of experiments on the photometrical value of sperm candles, during which, at my request, Mr. Hatcher was good enough to note the rate of combustion of such candles in a darkened room, and also in broad daylight, and even in sunshine.

In the first observation, three hard and three soft candles were burned, each for four hours, in a dark closet. A similar set of candles, taken from one and the same filling, were burned, during the same time, in open daylight. partly in sunlight. The average consumption per hour of each candle was as follows:

Sperm, in the dark.. Ditto, in the light No. 2 composites In the dark In the light It must be noticed that the temperature in the light was 72°, and in the dark 71°; moreover, in the light there was a much greater motion of the air than in the dark closet. Both these circumstances would operate in producing a larger consumption of candle.

In a second trial with No. 2 composites, the results

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hour, nearly.

It is evident that, in this case, the increase of temperature

caused by the bright sunshine led to an increased consumption of material.

It will be seen that, in the first and fourth trials, there is a greater consumption of material in the light than in the dark; and in the second and third trials, the consumption is greater in the dark than in the light; but, in any case, the difference is so small-amounting only to from 2 to 7 grains per hour-that it may fairly be referred to accidental circumstances, such as difference in temperature, in currents of air, and in the composition and make of the candles; the final conclusion to which I am led being that the direct light of the sun, or the diffused light of day, has no action on the rate of burning or in retarding the combustion of an ordinary candle.

SOME EXPERIMENTS WITH THE

GREAT INDUCTION COIL AT THE ROYAL POLYTECHNIC.*

By JOHN HENRY PEPPER, F.C.S., Assoc. Inst. C.E.

THE LARGE INDUCTION COIL.

THE length of the coil from end to end is 9 feet 10 inches, and the diameter 2 feet; the whole is cased in ebonite; it stands on two strong pillars covered with ebonite, the feet of the pillars being of a diameter of 22 inches. The ebonite tubes, &c. are the largest ever constructed by the Silver Town Works.

The total weight of the great coil is 15 cwts., that of the ebonite alone being 477 lbs.

The primary wire is made of copper of the highest conductivity, and weighs 145 lbs. ; the diameter of this wire is o'0925 of an inch, and the length 3770 yards. The number of revolutions of the primary wire round the core of soft iron is 6000, its arrangement being 3, 6, and 12 strands.

The total resistance of the primary is 2-201400 British Association units, and the resistances of the primary conductors are respectively-For three strands, 0733800 B.A.U.; six, o'366945 B.A.U.; twelve, o'1834725 B.A.U.

The primary core consists of extremely soft straight iron wires 5 feet in length, and each wire is o'0625 of an inch in diameter. The diameter of the combined wires is 4 inches, and the weight of the core is 123 lbs.

The secondary wire is 150 miles in length; it is covered with silk throughout, and the average diameter is 0.015 of

an inch.

The total weight of this wire is 606 lbs., and the resistance 33,560 B.A. units. The length of the secondary coil is 50 inches, and the insulation throughout is calculated to be 95 per cent beyond that required. The secondary wire is insulated from the primary by means of an ebonite tube of an inch in thickness and 8 feet in length.

The length of the secondary coil is 54 inches, the diameter is 19 inches, and without the internal ebonite tube containing the primary wire and iron core it is a cylinder 19 inches in diameter and 6 inches thick.

The condenser, made in the usual manner with sheets of varnished paper and tinfoil, is arranged in six parts, Abstract of a paper communicated to the Royal Society, by J. P. Gassiet, F.R.S.

NEWS

each containing 125 superficial feet, or 750 square feet of tinfoil in the whole.

A large and substantially made contact breaker, detached from the great coil and worked by an independent electromagnet, was constructed and worked very well with a comparatively moderate power of 10 or 20 large Bunsen's cells; when, however, the battery was increased to 30 or 40 cells, it became unmanageable.

A Foucault break, with the platinum amalgam and alcohol above it, was now tried, and answered very much better than the ordinary contact breaker; there was no longer any burning or destruction of the contact points, although the great power of the instrument appeared to cause continued decomposition in the water of the alcohol placed above the platinum amalgam; and every now and then the spirit was violently ejected, probably by explosion of the mixed gases taking place in the amalgam, in which they collected in bubbles; the alcohol took fire constantly, and had to be extinguished. A large and very strong glass vessel (in fact, an inverted glass cell belonging to a bichromate battery) was bored through, and the neck fitted into a cap with cement, a thick wire covered with platinum being inserted in the botton; the platinum amalgam was poured on this, and over it a pint or more of alcohol; the contact wire was also very thick and pointed with a thick stud of platinum, and, being attached to a spring, contact was easily made and broken. Explosions did not occur, flashes of light could be seen between the amalgam and the alcohol, and the height of the column of the latter prevented the forcible ejection of the spirit, which no longer took fire. The break was used for eight hours in a continuous series of experiments.

The Bunsen's battery used in the experiments was made with the largest porous cells that could be obtained, and each cell contained about one pint of nitric acid.

Some experiments were tried with the battery arranged for intensity, and used with the complete condenser of 750 square feet of tinfoil and 1500 square feet of paper. At first five cells were used, and these gave a spark 12 inches in length. The number of cells were gradually increased until 50 were in operation, when a spark from 28 to 29 inches in length was obtained.

In order to ascertain whether any variation in the size of the condenser would affect the length of the spark, a number of experiments were tried; and it was found that when half the condenser was used the spark increased in length up to twenty cells, but not after.

Experiments were now tried to ascertain whether any increase in the length of the spark could be obtained by arranging the battery and the primary coil for quantity, but no material advantage was obtained by this arrangement; even where three groups of cells were connected a decrease in the length of the spark is observed when compared with the 45 or 50 cells arranged for intensity, the difference being as 20 to 28.

The spark obtained from the large coil is thick and flame-like in its appearance, and therefore it will be alluded to as the "flaming spark."

When the discharging point and circular plate are brought within 6 or 7 inches of each other, the flaming nature of the spark becomes still more apparent.

Two light yellow flames curving upwards appear to connect the opposite poles. If a blast of air from powerful bellows is directed against a flaming spark, the flaming portion can be blown away and increased in area, and thin wiry sparks are now seen darting through it, sometimes in one continuous stream, at another time divided into three or more sparks, all following the direction in which the flame is blown.

The flaming spark is very hot, and if passed through asbestos (supported on an insulating pillar), quickly causes the latter to become red-hot.

When powdered charcoal is shaken from a pepper-box into the flaming spark in a vertical line and in consider able quantities, the greater part of the light is obscured, and the whole form of the flaming spark presents the ap

pearance of a black cloud with a line of brightly ignited particles fringing the bottom parts. If the charcoal is dusted through in small quantities, each particle becomes ignited, like blowing charcoal into a hydrogen flame.

When the flaming spark is directed on to a glass plate upon which a little solution of lithium chloride is placed, the latter colours the flame upwards to the height of 3 or 4 in the most beautiful manner; and if the point of the discharge is tipped with paper, or sponge moistened with a little solution of sodium chloride, the two colours (the yellow from the salt, and the crimson from the lithium) meet each other, a neutral point being found about halfway, and thus illustrating apparently the dual character of electricity, and that + passes to electrical, and vice versâ.

The flaming spark can be obtained in perfectly dry air. Whilst passing through common air, if blown against a sheet of damp litmus-paper, the latter is rapidly changed red. In order to ascertain whether the acid product was nitric acid, the flaming spark (9 or 10 inches in length) was passed through a tube connected by a cork and bent tube with a bottle containing distilled water, from which another tube passed to the air-pump; on drawing the air slowly over the spark, and passing the former into the bottle, nitric acid was obtained in large quantities, so much so that it could be detected by the smell and taste as well as by the ordinary tests. The popular notion that nitric acid is always produced during a thunder-storm would therefore appear to be correct. To determine the effect of a cooling surface on the flaming spark, a hole 1 inches in diameter was bored through a thick block of Wenham Lake ice, and the spark passed through the air in the tube of ice; no change took place, and the spark was still a flaming one.

When the spark was received on the ice, it lost its flaming character, and became thin and wiry, spreading out in all directions.

If the discharging wires were tipped with ice, the spark was always flaming when any thickness of air intervened between them. Even over the ice, if the spark passed a fraction of an inch above the surface, it was always a flaming one, but changed to the thin spark when the point of the discharging wire was thrust into the ice.

If one of the discharging wires of the great coil is brought to the centre of large swing looking-glass and the other wire connected with the amalgam at the back, the sparks are thin and wiry, arborescent, and very bright; the crackling noise of these discharges being quite different from that of the heavy thud or blow delivered by the flaming spark.

When the discharging wire is brought close to the flame of the looking-glass, or if a sufficient thickness of air intervenes, the spark again becomes flaming; or, as sometimes occurs, if the discharging wire is placed about 5 inches from the frame, the spark is partly flaming and partly wiry, i. e., when it impinges on the glass.

The spectrum is a continuous one with the sodiumline.

When the blast of air is used, and the wiry sparks made apparent, then the nitrogen line appears.

The flaming spark has been ascribed by some experienced observers to the incandescence of the dust in the air, and especially sodium chloride.

To ascertain whether the "flaming spark" could be obtained with a small number of cells, the large Bunsen's battery was reduced to three cells, and it was found that no appreciable spark could be produced when the whole primary wire was used with less than five cells.

By reducing the length of the primary wire, and using the four divisions separately, with five cells the spark was wiry, and varied from 44 inches to 6 inches; with ten cells it was wiry, and varied from 8 to 9; in the latter the spark was slightly flaming. With fifteen cells the spark was slightly flaming, and varied from 10 inches to 11 inches. With 20 cells a flaming spark varying from 11 inches to 12 inches was obtained.

* p (0)