H

"The action of the instrument is as follows. The standard lamp being placed on one of the supporting pillars which slide along the graduated stem (Fig. 17), it is adjusted to the proper height, and moved along the bar to a convenient distance, depending on the intensity of the light to be measured; the whole length being a little over 4 ft., each light can be placed at a distance of 24 in. from the disc. The flame is then sheltered from currents of air by black screens placed round, and the light to be compared is fixed in a similar way on the other side of the instrument. The whole should be placed in a dark room, or surrounded with non-reflecting screens; and the eye must also be protected from direct rays from the two lights. On looking through the eye-piece two bright discs will be seen, probably of different colours. Supposing E represents the standard flame, and F the light to be compared with it, the latter must now be slid along the scale until the two discs of light, seen through the eyepiece, are about equal in tint. Equality of illumination is easily obtained; for, as the eye is observing two adjacent discs of light, which pass rapidly from red-green to green-red, through a neutral point of no colour, there is no difficulty in hitting this point with great precision. It has been found most convenient not to attempt to get absolute equality in this manner, but to move the flame to the nearest inch on one side or the other of equality. The final adjustment is now effected at the eye-end, by turning the polarimeter one way or the other up to 45°, until the images are seen without any trace of colour. This will be found more accurate than the plan of relying entirely on the alteration of the distance of the flame along the scale; and, by a series of experimental adjustments, the value of every angle through which the bundle of plates is rotated can be ascertained once for all, when the future calculations will present no difficulty. Squaring the number of inches between the flames and the centre will give their approximate ratios; and the number of degrees the eye-piece rotates will give the number to be added or subtracted in order to obtain the necessary accuracy.

K

M

FIG. 18.

"The delicacy of the instrument is very great. With two lamps, each about 24 in. from the centre, it is easy to distinguish a movement of one of them to the extent of 1-10th of an inch to or fro; and by using the polarimeter, an accuracy considerably exceeding that can be attained.

"The employment of a photometer of this kind enables us to compare lights of different colours with one another, and leads to the solution of a problem which, from the nature of their construction, would be beyond the powers of the instruments in general use. So long as the observer, by the eye alone, has to compare the relative intensities of two surfaces respectively

illuminated by the lights under trial, it is evident that unless they are of the same tint it is impossible to obtain that absolute equality of illumination in the instrument which is requisite for a comparison. By the unaided eye one cannot tell which is the brighter half of a paper disc illuminated on one side with a reddish, and on the other with a yellowish light; but, by using the above-described photometer, the problem becomes practicable. For instance, on reference to Fig. 16, suppose the disc D were illuminated with light of a reddish colour, and the disc c with greenish light, the polarized discs ad would be reddish, and the discs c' c greenish, the central disc c d being of the tint formed by the union of the two shades. The analysing prism K, and the selenite disc I, will detect free polarization in the disc c d, if it be coloured, as readily as if it were white; the only difference being that the two discs of light gr cannot be brought to a uniform white colour when the lights from D and C are equal in intensity, but will assume a tint similar to that of cd. When the contrasts of colour between D and C are very strong-when, for instance, one is a bright green and the other scarlet-there is some difficulty in estimating the exact point of neutrality; but this only diminishes the accuracy of the comparison, and does not render it impossible, as it would be according to other systems.

"No attempt has been made in these experiments to ascertain the exact value of the standard spirit-flame in terms of the Parliamentary sperm candle. Difficulty was experienced in getting two lots of candles yielding light of equal intensities; and when their flames were compared between themselves and with the spirit-flame, variations of as much as 10 per cent. were sometimes observed in the light they gave. Two standard spirit-flames, on the other hand, seldom showed a variation of 1 per cent., and had they been more carefully made they would not have varied o'i per cent.

"This plan of photometry is capable of far more accuracy than the present instrument will give. It can scarcely be expected that the first instrument of the kind, roughly made by an amateur workman, should possess equal sensitiveness with one in which all the parts have been skilfully made with special adaptation to the end in view."

MODIFICATIONS THAT LIGHT MAY UNDERGO.

I. In the same medium of the same density rays of light undergo no change.

2. When rays of light pass out of one medium into another, or into one of a different density, they may undergo the following modifications:

3. The rays of light may rebound from the surface of a solid, fluid, or gaseous body, and are then said to be reflected, the rebounding being denominated Reflection.

4. A ray of light, after passing into a substance, may be bent from its natural course, or Refracted.

5. A ray of light may be split into two portions when it enters certain bodies, such as Iceland spar, and each portion of the light possesses distinct properties.

6. A ray of light may be so checked in its passage that a portion may be lost or absorbed.

7. A ray of light, by reflection, refraction, double refraction, and absorption, may acquire new properties, and become what is termed Polarized Light.

THE REFLECTION OF LIGHT.

Catoptrics is the name given to all effects produced by reflection. It is a word taken from the Greek каTоTTрikos, "belonging to a mirror," and whilst the laws which govern the reflection of light are remarkably simple, they give rise to a most interesting series of phenomena.

Premising that the incident rays

are those which fall on the surface, and that those sent off are called reflected rays, it is soon ascertained -Ist, that the incident and reflected rays always lie in the same plane, i.e., if the incident ray falls in a perpendicular plane or direction, the reflected one will also be in the same plane or direction; and the like reasoning applies to the horizontal position. 2nd, the incident and reflected rays always form equal angles, or when light falls upon any surface, whether plane or curved, the angle of reflection is equal to the angle of incidence. The luminous rays may be parallel to each other, like the lines in a copybook, or they may be divergent when they spread out in the same manner as the sticks of a fan, or convergent when they gradually approach each other, and end in a point like a spear-head.

FIG. 19.

R R is the reflecting surface; A B is the incident ray;
BC, the reflected ray; A B P, the angle of incidence;

CB P, the angle of reflection.

FIG. 20.-Reflection of Parallel or Equi-distant Rays. RRR, the parallel rays incident on a plane or flat surface at T, and reflected in lines at equal distances from each other. The rays of the sun are nearly parallel with each other, and will illustrate this fact.

M

M

FIG. 21.

Parallel rays falling on a concave mirror, M Mм, converge or come to a focus or fireplace at F.

Reflection of parallel rays from a convex mirror. The rays which are reflected become divergent,

A very large number of the waves of light are lost when they fall even upon the most perfectly polished metallic mirrors; thus light reflected from a clear and bright surface of metallic mercury at an angle of 78° 5' loses nearly one quarter, and only 754 rays out of 1000 are reflected.

A transparent substance, like glass, reflects more light from the second surface than the first; and if the former is coated with an amalgam of tin and mercury, the brilliancy of the reflection of the second or coated surface overpowers that of the first, although if a candle is held opposite the best quicksilvered mirror two images are apparent.

R

B

In the production of illusory effects by reflection from the surface of glass, the image reflected from the surface of the first surface interferes with the second; but this may be prevented, as shown to the author by a friend, by coating the first side with a very delicate film of collodion or varnish, such as is used for photographic purposes. Thus the intensity of the reflection of the second surface is increased by a coating of amalgam, whilst the intensity of the reflection from the first surface is reduced by coating it with a substance like collodion, having an absorptive rather than a reflecting power on light. Where objects are reflected from either glass or silvered glass plane mirrors, they appear to come from the back, and the image is as far behind the glass as the real object is before it. It is this physical truth that increases so amazingly the effect of what is familiarly called "The Ghost Illusion." The spectator looking at the image does not observe the glass which has produced it, because the former is so far in advance of the latter. Had this physical fact in catoptics been remembered, many scientific men would have sooner discovered the secret of the illusion by looking in front of the image for the glass or reflecting surface.

FIG. 23.

o, the real object reflected from the glass A B,.at R, to the eye at E; so, behind the glass, is where the image appears to come from, and if the whole distance from E to so is measured, it will be found equal to E R, R o.

The same truth is still more apparent when divergent or convergent rays are traced out in their reflections from a plane surface of glass.

To cause the image or ghost to appear, the lights are alternately thrown on or cut off the real figure. (See Fig. 24, p. 24.)

This mode of showing the ghost has to be modified when the angles of vision are so different as seen from the pit, boxes, and gallery of a theatre. Then it is advisable to sink a stage a few feet below the regular stage, and to arrange a board at the same angle as the glass, on which the living figures recline. The latter method allows only certain movements to be exhibited, and is called the "spectroscope" and "phantoscope" by travelling showmen who exhibit the ghost.

One of the prettiest stories which can be illustrated with this illusion is that called "The Knight watching his Armour," and as many persons have