or if a sensitive photographic plate, capable of being affected by light, be introduced instead of the ground glass, we shall be able to obtain an impression of the image upon it, and it is thus that portraits are taken by photography. It will be seen that the various objects whose images we thus obtain should be as nearly as possible at the same distance from the lens, so that all their images may all attain their greatest distinctness on the same plate, BC; for if an object be far off, its image will be brought to a focus nearer the lens than the image of an object close at hand.

281. The Eye.—The eye may be compared to a camera such as we have now described. The front part of it contains a lens, and there is a variable aperture admitting light, which is called the pupil. The pupil adapts itself in size according to the amount of light; if the field around be very luminous the pupil contracts, so as to diminish the light which enters the eye; but if there be little light it dilates. Now, just as in the camera an image of the objects in front is formed on a plate, so in the eye an image of the surrounding objects is formed on a membrane in the back part of the eye called the retina, and this is connected with a nerve called the optic nerve, which finally conveys the impression to the brain.

The eye has a considerable power of adjustment for different distances, so that if it be viewing a near object, the image of which is thrown exactly on the retina, and if then all at once a distant object be viewed, by an exercise of this power the eye is able to adjust itself so as to throw the image exactly on the retina. But sometimes when the lens of the eye is too convex, distant objects will not be distinctly seen, for the focus of their image will be in front of the retina, and only near objects will be distinctly seen; a person in such a case is called short-sighted. This defect is obviated by the use of spectacles formed of diverging lenses, which serve to correct the excessive convergence of the lens of the eye.

On the other hand, the lens of the eye is sometimes not convex enough, so that while the images of distant objects are

thrown upon the retina, those of near objects cannot be brought to a focus within the eye. In such a case the individual is said to be long-sighted, and the defect is obviated by the use of convex glasses, to increase the converging of the eye. power

282. Simple Microscope.-An ordinary convex lens may be used as a means of magnifying small bodies, and its action in this respect will be seen with reference to Fig. 89. Here the eye is placed, let us say, at the point E, and the object to be viewed is placed at AB, somewhere between the lens and its principal focus; the result is that we have an enlarged virtual image of the object, as if it were placed at A'B', and this image will be perceived by the eye at E. This virtual image must not, however, be too near the eye, otherwise the eye will not perceive it distinctly; for experience must convince us that an object placed too close to the eye will not be distinctly seen. The virtual image ought not to be nearer the eye than about ten inches or a fcot.

283. Telescope.-A telescope for observing a star or distant object consists essentially of two lenses, one an objectglass, and the other an eye-piece. Thus, in Fig. 91, let A B

be the distant object which we are viewing, and o the objectglass. This glass will give an image of A B at its principal focus. Let us call this image a b. We have now only to view the image ab through a simple microscope or eye-piece, just as if it were a real body, and we shall thus obtain a virtual and magnified image A'B' of the distant object.

Thus the difference between the simple microscope and the telescope is, that in the former we scrutinize by a magnifier the object itself, while in the latter we scrutinize the image of the object.

LESSON XXXI. - DISPERSION OF LIGHT BY THE PRISM.

284. We have hitherto dealt with those properties which are common to all kinds of luminous rays, whether they be white or green or blue or red, but we now come to certain characteristics which enable us to distinguish between different kinds of light.

In the first place, it is necessary to know that those bodies with which we are most familiar give out light, which consists of a great many different kinds all blended together. Therefore, before we can thoroughly examine the character of the light given out by any hot substance, we must devise some means of sifting or separating these various rays from one another, so as to know how many individual rays we have, and what is the intensity of each.

We are all familiar with the magnificent display of colours exbibited by gems, when rays of light are allowed to fall upon them in a particular direction. On such occasions they sparkle with all the colours of the rainbow, and this very allusion bids us ask if the hues of the rainbow be not due to the same cause as the colours of gems. Does not the very name imply the presence in the sky of a multitude of minute spheres of water, such as would on the grass shine forth like innumerable diamonds? Are not all these displays due to the same cause; and if so, what is the cause? The discovery of it is due to Newton, who was the first to show that white light is in reality composed of a great many differently coloured rays, and that these rays are, in their passage through transparent substances, in certain cases separated from each other.

285. The prism gives us the means of separating the variously coloured constituents of a compound ray from one another.

Suppose, for instance, that we have a narrow vertical slit in the shutter of a dark room, through which white sunlight is allowed to pass. Fig. 92 will represent a ground plan of this arrangement; while the prism employed is represented in elevation in Fig. 92a.

If we look towards the slit from E without a prism, we shall see it lit up in the usual manner; in fact it will serve as an opening through which we may see the sun beyond. Let us now, however, interpose a vertical glass prism between our eye and the slit. When we have done so, the slit will no longer be visible. If, however, we place our eye somewhere above E (as regards the diagram) we shall now see the light from the slit. But it will not reach us in the shape of a luminous slit as formerly, but will appear as a broad band or ribbon of light of many colours, beginning with red at the one end, and passing gradually and successively through orange, yellow, green, blue, and indigo, to violet at the other extremity.

286. All this may be very easily explained. In the first place, we have already remarked (Art. 276) that rays of light are bent in their passage through the prism, so that the eye, which was formerly placed at E, must now be placed above it in order to see the slit. If all the rays were equally bent by the prism we should still see the slit as before, the only change being one of direction; but the great importance of the experiment consists in the fact that all the rays are not equally bent as they pass through the prism, but rays of one colour are bent differently from those of another colour. This

different bending of the various rays is termed their dispersion.

If the ray be a red one, it will emerge from the prism bent to a certain extent; if a yellow ray, it will be somewhat more bent; if green, still more, and so on.

If the light which streams through the slit be compounded of many-coloured rays, all the various compounds of this light may be viewed separately by means of the peculiar dispersive action of the prism which we have now described.

If, however, the slit be wide, its position due to one ray will overlap that due to another ray, and the result will be a certain blending together of different rays, and consequent imperfection of the method as far as regards separation of the various constituents of the light of the slit. It is therefore of great importance in all such investigations to make use of a very narrow slit.

It ought also to be borne in mind that the rays of light which have been separated to a certain extent by one prism will be still more separated by a second one, applied in a proper manner behind the first, and that we shall by this ineans obtain increased separation for each additional prism which we use.

287. An instrument furnished with prisms for analysing rays of light is called a Spectroscope, and the following plate gives a representation of a very powerful instrument of this description belonging to the late Mr. J. P. Gassiot.

The slit to be illuminated by the light we wish to analyse is at the extreme left, the telescope to which it is attached being called the collimator, and its object being to reduce the divergent rays from the slit into parallelism before they fall upon the train of prisms, as it is only in this state of things that we shall get a good result.

The telescope to the right is an ordinary magnifying telescope, intended to magnify the various images of the slit due to the various constituent rays that have passed through the train of prisms.

By an instrument such as this, we may transform the light of the sun illuminating the slit into a broad, many-coloured

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