Part 14 (1/2)

Chapter XII

OPTICS

Lenses--The ie cast by a convex lens--Focus--Relative position of object and lens--Correction of lenses for colour--Spherical aberration--Distortion of ie--The huht is a third fory of which we have already treated twocharacteristic of ether light-waves is their extree from 700 billion movements per second for violet rays to 400 billion for red rays

If a beah a prism it is resolved into the seven visible colours of the spectrue, and red--in this order The huraphic plate to the green-violet rays

All bodies fall into one of two classes--(1) _Luht, such as the sun, a candle flame, or a red-hot coal; and (2) _non-luht which they receive from other bodies and reflect to our eyes

THE PROPAGATION OF LIGHT

Light naturally travels in a straight line It is deflected only when it passes from one transparent medium into another--for example, from air to water--and the ard the surface of a visible object aspencil of rays is sent off through the ether

LENSES

If a bealass body with non-parallel sides, the rays are deflected The direction they take depends on the shape of the body, but it may be laid down as a rule that they are bent toward the thicker part of the glass The co the sun to concentrate all the heat rays that fall upon it into one intensely brilliant spot, which speedily ignites any inflaine that one ray passes frolass This is undeflected; but all the others are bent towards it, as they pass through the thinner parts of the lens

[Illustration: FIG 103--Showing how a burning-glass concentrates the heat rays which fall upon it]

It should be noted here that _sunlight_, as we call it, is accolass is used to concentrate the _heat_ rays, not the _light_ rays, which, though they are collected too, have no igniting effect

In photography we use a lens to concentrate light rays only Such heat rays as h the lens with them are not wanted, and as they have no practical effect are not taken any notice of To be of real value, a lens must be quite symmetrical--that is, the curve from the centre to the circumference must be the same in all directions

There are six for 104 Nos 1 and 2 have one flat and one spherical surface Nos 3, 4, 5, 6 have two spherical surfaces When a lens is thicker at the middle than at the sides it is called a _convex_ lens; when thinner, a _concave_ lens The names of the various shapes are as follows:--No 1, plano-convex; No 2, plano-concave; No 3, double convex; No 4, double concave; No 5, meniscus; No 6, concavo-convex The thick-centre lenses, as we may term the through them; while the thin-centre lenses (Nos 2, 4, 6) _scatter_ the rays (see Fig 105)

[Illustration: FIG 104--Six forms of lenses]

THE CAMERA

[Illustration: FIG 105]

[Illustration: FIG 106]

We said above that light is propagated in straight lines To prove this is easy Get a piece of cardboard and prick a hole in it Set this up some distance away from a candle flame, and hold behind it a piece of tissue paper You will at once perceive a faint, upside-down ie of the fla 106, which shows a ”pinhole” calass screen, B, to catch the ie Suppose that A is the lowest point of the fla from it strikes the front of the camera, which stops theh the hole and makes a tiny luh A is below the axis of the camera Similarly the tip of the flame (above the axis) would be represented by a dot on the screen below its centre And so on for all the e the hole we should get a brighter ie, but it would have less sharp outlines, because a number of rays from every point of the candle would reach the screen and be juh a good, sharp photographthat photography of this sort has little practical value What ant is a large hole for the light to enter the cae If we place a lens in the hole we can fulfil our wish

Fig 107 shows a lens in position, gathering up a nu them on a point, B If the lens has 1,000 times the area of the pinhole, it will pass 1,000 tie of A will be iraphic plate 1,000 times more quickly

[Illustration: FIG 107]

THE IMAGE CAST BY A CONVEX LENS