第28章

is 25, that of the calorific spectrum will be 42.10, and of the chemical spectrum 55.10. Such a series of circles may well be used to represent a beam from the sun, which may be regarded as an atom of Light, surrounded with an invisible atmosphere of Heat, and another still more extended, which possesses the remarkable property of producing chemical and molecular change.

A ray of light, in passing obliquely through any medium of uniform density, does not change its course; but if it should pass into a denser body, it would turn from a straight line, pursue a less oblique direction, and in a line nearer to a perpendicular to the surface of that body.

Water exerts a stronger refracting power than air; and if a ray of light fall upon a body of this fluid its course is changed, as may be seen by reference to Fig. 4.

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It is observed that it proceeds in a less oblique direction (towards the dotted line), and, on passing on through, leaves the liquid, proceeding in a line parallel to that at which it entered. It should be observed that at the surface of bodies the refractive power is exerted, and that the light proceeds in a straight line until leaving the body.

The refraction is more or less, and in all cases in proportion as the rays fall more or less obliquely on the refracting surface.

It is this law of optics which has given rise to the lenses in our camera tubes, by which means we are enabled to secure a well-delineated representation of any object we choose to picture.

When a ray of light passes from one medium to another, and through that into the first again, if the two refractions be equal, and in opposite directions, no sensible effect will be produced.

The reader may readily comprehend the phenomena of refraction, by means of light passing through lenses of different curves, by reference to the following diagrams:--[amdg_5.gif]

Fig 5 represents a double-convex lens, Fig. 6 a double-concave, and Fig.

7 a concavo-convex or meniscus. By these it is seen that a double-convex lens tends to condense the rays of light to a focus, a double-concave to scatter them, and a concavo-convex combines both powers.

If parallel rays of light fall upon a double-convex lens, D D, Fig.

8, they will be refracted (excepting such as pass directly through the centre)to a point termed the principal focus.

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The lines A B C represent parallel rays which pass through the lens, D D, and meet at F; this point being the principal focus, its distance from the lens is called the focal length.

Those rays of light which are traversing a parallel course, when they enter the lens are brought to a focus nearer the lens than others. Hence the difficulty the operator sometimes experiences by not being able to "obtain a focus,"when he wishes to secure a picture of some very distant objects;he does not get his ground glass near enough to the lenses.

Again, the rays from an object near by may be termed diverging rays.

This will be better comprehended by reference to Fig.

9, where it will be seen that the dotted lines, representing[amdg_9.gif]

parallel rays, meet nearer the lenses than those from the point A. The closer the object is to the lenses, the greater will be the divergence.

This rule is applicable to copying. Did we wish to copy a 1/6size Daguerreotype on a l/l6 size plate, we should place it in such a position to the lenses at A that the focus would be at F, where the image would be represented at about the proper size.

Now, if we should wish to copy the 1/6 size picture, and produce another of exactly the same dimensions, we have only to bring it nearer to the lenses, so that the lens D E shall be equi-distant from the picture and the focus, i. e. from A to B. The reason of this is, that the distance of the picture from the lens, in the last copy, is less than the other, and the divergence has increased, throwing, the focus further from the lens."These remarks have been introduced here as being important for those who may not understand the principles of enlarging or reducing pictures in copying.

I would remark that the points F and A, in Fig.

9, are termed "conjugate foci."

If we hold a double-convex lens opposite any object, we find that an inverted image of that object will be formed on a paper held behind it.

To illustrate this more clearly, I will refer to the following woodcut:

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