Part 16 (1/2)

[Illustration: FIG 125]

THE GALILEAN TELESCOPE

[Illustration: FIG 126]

A third forreat Italian astrono 126

The rays trans to a focus, on a concave lens which separates theence denoted by the dotted lines The ilasses are constructed on the Galilean principle

THE PRISMATIC TELESCOPE

In order to be able to use a long-focus object-glass without a long focussing-tube, a syste pris through the object-glass is reflected from one posterior surface of prish the front on to a second prisles to it, which passes the ray on to the colass and eye-piece is thus practically trebled The best-known prislasses

[Illustration: FIG 127]

THE REFLECTING TELESCOPE

We ely used by astrono no object-glass Rays from the object fall on a parabolic mirror situated in the rear end of the tube This reflects them forwards to a focus In the Newtonian reflector a plane mirror or prism is situated in the axis of the tube, at the focus, to reflect the rays through an eye-piece projecting through the side of the tube Herschel's forle to the axis, so that the rays are reflected direct into an eye-piece pointing through the side of the tube towards the128) is of such a shape that all rays parallel to the axis are reflected to a coht a powerful arc laed with the arc at the focus of a parabolic reflector, which sends all reflected light forward in a pencil of parallel rays The ht equal to that of 350 million candles

[Illustration: FIG 128--A parabolic reflector]

THE COMPOUND MICROSCOPE

We have already observed (Fig 110) that the nearer an object approaches a lens the further off behind it is the real ie formed, until the object has reached the focal distance, when no ie at all is cast, as it is an infinite distance behind the lens We will assume that a certain lens has a focus of six inches We place a lighted candle four feet in front of it, and find that a _sharp_ dilass screen held seven inches behind it Ifexchange the positions of the candle and the screen, we shall get an enlarged ie of the candle This is a siate foci_--namely, that the distance between the lens and an object on one side and that between the lens and the corresponding ie on the other bear a definite relation to each other; and an object placed at either focus will cast an ier or smaller than the object depends on which focus it occupies In the case of the object-glass of a telescope the ie was at e may call the _short_ focus

[Illustration: FIG 129--Diagram to explain the compound microscope]

Now, a compound microscope is practically a telescope with the object at the _long_ focus, very close to a short-focus lens A greatly enlarged iate focus, and this is caught and still further lass, or _objective_, of a microscope is usually compounded of several lenses, as is also the eye-piece

THE MAGIC-LANTERN

The ic-lantern are:--(1) The _source of light_; (2) the _condenser_ for concentrating the light rays on to the slide; (3) the _lens_ for projecting a130 shows these diagrammatically The _illuas jet, or a cylinder of lien flareat heat, and when the supply of oxygen is artificially increased the temperature of the flaen jet has an interior pipe connected with the cylinder holding one gas, and an exterior, and so the other, the two being arranged concentrically at the nozzle By ulated to give the best results

[Illustration: FIG 130--Sketch of the eleic-lantern]

The _condenser_ is set somewhat further froth of the lenses, so that the rays falling on them are bent inwards, or to the slide

The _objective_, or object lens, stands in front of the slide Its position is adjustable by ht to the slide the further away is the conjugate focus (see p