Part 16 (2/2)

239), and consequently the ie The exhibitor first sets up his screen and lantern, and then finds the conjugate foci of slide and i the lens in or out

If a very short focus objective be used, subjects of microscopic proportions can be projected on the screen enore of Paris in 1870-71 the Parisians established a balloon and pigeon post to carry letters which had been copied in a raphy These copies could be enclosed in a quill and attached to a pigeon's wing On receipt, the copies were placed in a special lantern and thrown as large writing on the screen Micro-photography has since then reat strides, and is noidely used for scientific purposes, one of thethe study of the crystalline formations of metals under different conditions

THE BIOSCOPE

”Living pictures” are the ic-lantern entertainatives fro a ribbon of sensitized celluloid through a special form of camera, which feeds the ribbon past the lens in a series of jerks, an exposure beingeach rest The positive film is placed in a lantern, and the intermittent movement is repeated; but now the source of illuht passes outwards through the shutter to the screen In the Urban bioscope the film travels at the rate of fifteenmade every second

The impression of continuous et rid of a visual impression in less than one-tenth of a second So that if a series of impressions follow one another more rapidly than the eye can rid itself of theive one of _motion_, if the position of sohtly in each succeeding picture[25]

THE PLANE MIRROR

[Illustration: FIG 131]

This chapter lass Why do we see a reflection in it? The answer is given graphically by Fig

131 Two rays, A _b_, A _c_, from a point A strike the mirror M at the points _b_ and _c_ Lines _b_ N, _c_ O, drawn from these points perpendicular to the les A _b_ N, A _c_ O are the _angles of incidence_ of rays A _b_, A _c_ The paths which the rays take after reflection les with _b_ N and _c_ O respectively equal to A _b_ N, A _c_ O These are the _angles of reflection_ If the eye is so situated that the rays enter it as in our illustration, an ie of the point A is seen at the point A1, in which the lines D _b_, E _c_ meet when produced backwards

[Illustration: FIG 132]

When the verticalsurface, such as a pond (Fig 132), the sa happens The point at which the ray from the reflection of the spire's tip to the eye appears to pass through the surface of the water must be so situated that if a line were drawn perpendicular to it froles made by lines drawn from the real spire tip and from the observer's eye to the base of the perpendicular would be equal

[22] Glazebrook, ”Light,” p 157

[23] Glazebrook, ”Light,” p 157

[24] Galileo was severely censured and i to maintain that the earth moved round the sun, and revolved on its axis

[25] For a full account of Anieously consult ”The Romance of Modern Invention,” pp 166 foll

Chapter XIV

SOUND AND MUSICAL INSTRUMENTS

Nature of sound--The ear--Musical instru-board and the fra ht, heat, and electricity in that it can be propagated through matter only Sound-waves arean electric bell under the bell-glass of an air-pu all the air Ether still relass, but if the bell be set in ong is heard quite plainly

Sound reseht and heat, however, thus far, that it can be concentrated by means of suitable lenses and curved surfaces An _echo_ is a proof of its _reflection_ fro with the various appliances used for producing sound-waves of a definite character, let us examine that wonderful natural apparatus