the ordinates give the intensity of the light in terms of that of the standard.

By means of an independent series of observations the wave-length of the light which falls on the slit in any given position can be found, as in § 62, and thus a curve giving intensity in terms of wave-length can be determined. This curve is called a luminosity curve. The form of the curve, as found by Captain Abney, is given in fig. xxxiv. The

FIG. XXXIV.

WL4000

F

5000 E

D 6000 C B 7000

measurements are to some extent affected by the colour of the receiving screen; a card painted with two coats of zinc oxide gives the best results. A portion of this screen about 5 cm. square, limited by a sheet of black paper with a hole cut in it, should be used.

Instead of moving the standard light, the method of varying its intensity adopted by Captain Abney in some later. experiments may be employed (' Proc. R. S.' vol. xliii. p. 249).

A circular disc is placed between the standard light and the screen. The disc is divided into four quadrantal sectors, and the alternate sectors are removed. If such a disc is rotated between the light and the screen, it is clear that half the light is cut off. To the disc a pair of movable sectors are fitted, and these can be adjusted so as to close

to a greater or less extent, as may be required, the open sectors of the main disc. If, for example, the open sectors be half closed by the adjustable sectors, the transmitted light has only half the intensity of that previously transmitted.

By means of suitable mechanism the position of these movable sectors can be adjusted relatively to the others while the apparatus is in motion, and thus the amount of light from the standard can be varied until the luminosity of the shadows is the same. In this method of making the observations the slit is fixed in position and the sectors adjusted. When the adjustment has been made the motor is stopped, and the position of the sectors determined; from this the intensity of the standard can be found.

The apparatus can be used to examine the effect of colour mixtures by placing two or more slits in the screen D D. A coloured image of the face of the prism will be formed by light passing through each slit, and these images are superposed. By opening each slit in turn and finding the luminosity, and then making measurements with the two or three slits open simultaneously, we can verify the law that the impression due to a mixed light is the sum of the impressions due to each light separately.

The apparatus has been employed by Captain Abney to study colour-blindness, by comparing the luminosity curves found by various observers, and also for experiments on the scattering of light by small particles. For this purpose a glass trough filled with pure water was placed between the source and the slit s, and the luminosity curve found. Then a solution of mastic in alcohol was dropped in various quantities into the water, and the curve again determined. It was found that the intensity of the transmitted light was very closely in accordance with the formula found by Lord. Rayleigh, in accordance with which

Io being the intensity of the incident light, x the thickness of the absorbing medium, & a constant, and X the wavelength.

Experiments.

(1) Determine the luminosity curve for the various components of the light from the given source, and compare the result with the normal curve.

(2) Shew that the intensity of a mixture of colours is the sum of the intensities of the components.

(3) Determine the absorption in different parts of the spectrum produced by the given solution of mastic, and compare your result with Lord Rayleigh's formula.

The curve can be drawn from these.

(2) Slits were placed in the red, green, and violet, and the

luminosities observed for each slit separately, and for the slits

in pairs, and also all three together. The corresponding values were calculated from the curve on the assumption that the resulting impression is the sum of the individual ones.

(3) The intensity for various wave-lengths before and after absorption was determined. The table gives the observed and calculated ratio:

CERTAIN bodies, as, for instance, the iron ore called lodestone, and pieces of steel which have been subjected to certain treatment, are found to possess the following properties, among others, and are called magnets.

If a magnet be suspended at any part of the earth's surface, except certain so-called magnetic poles, so as to be free to turn about a vertical axis, it will in general tend to set itself in a certain azimuth-i.e. with any given vertical plane, fixed in the body, inclined at a certain definite angle to the geographical meridian-and if disturbed from this position will oscillate about it.

If a piece of iron or steel, or another magnet, be brought

FF

near to a magnet so suspended, the latter will be deflected from its position of equilibrium.

If a magnet be brought near to a piece of soft iron or unmagnetised steel, the iron or steel will be attracted by the magnet. This is illustrated by the experiment of § S, p. 467.

If a long thin magnetised bar of steel be suspended so as to be capable of turning about a vertical axis through its centre of gravity, it will be found to point nearly north and south. We shall call the end which points north the north end of the magnet, the other the south end.

DEFINITION OF UNIFORM MAGNETISATION.-If a magnet be broken up into any number of pieces, each of these is found to be a magnet. Let us suppose that the magnet can be divided into a very large number of very small, equal, similar, and similarly situated parts, and that each of the parts is found to have exactly the same magnetic properties. The magnet is then said to be uniformly magnetised.

DEFINITION OF MAGNETIC AXIs of a Magnet.-If any magnet be supported so as to be free to turn in any direction about its centre of gravity, it is found that there is a certain straight line in the magnet which always takes up a certain definite direction with reference to the earth. This line is called the magnetic axis of the magnet.

DEFINITION OF MAGNETIC MERIDIAN.-The vertical plane through this fixed direction is called the plane of the magnetic meridian.

DEFINITION OF MAGNETIC POLES.-If the magnet be a long thin cylindrical bar, uniformly magnetised in such a way that the magnetic axis is parallel to the length of the bar, the points in which the axis cuts the ends of the bar are the magnetic poles. The end of the bar which tends to point north, when the magnet is freely suspended, is the north, or positive pole; the other is the south, or negative pole. Such a magnet is called solenoidal, and behaves to other magnets as if the poles were centres of force, the rest of the magnet being devoid of magnetic action. In all actual