1018SCHOOL SCIENCE AND MATHEMATICS

instructor ’at a prearranged period during troublesome times.3. Notebooks. In previous classes it was clearly demonstrated

that the best pupils were those who maintained one or two wellorganized notebooks in addition to the laboratory notebook. Specialsuggestions, explanations, notes from the board, etc., could be foundin them. Notebooks are very helpful for reviews.

SOME LECTURE DEMONSTRATIONS IN LIGHT.By R. C. COLWELL,

Department of Physics, West Virginia University.With two large spherical mirros about three feet in

diameter such as are used in search lights, it is possibleto show several striking experiments in radiation to alarge class. First of all the mirrors are set up at oppositeends of the lecture desk about thirty feet apart and astick of wood is ignited in the focus of one mirror by theheat from an arc lamp in the focus of the other. (Figure1.) By scattering chalk dust in the path of the light rays,

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^�~"^~7^-FIG. 1. IGNITING A PIECE OF WOOD AT A DISTANCE

OF THIRTY FEET FROM A CARBON ARC.

one may show the direction of the rays for different posi-tions.of the electric arc. If one of the mirrors is set upona rotating stool and a red electric light placed in front ofit; then for the proper conditions, the whole class can seethe real inverted image and the enlarged virtual .image.At the same time the equation ^ -|- -^- = ^- is discussedin relation to the images. The physical fact that theangle of incidence is equal to the angle of reflectionmay be demonstrated bymeans of the arrangementshown in Figure 2. ANernst glow lamp is placedbehind a suitable collimatorand lens so as to give aparallel beam of light. The

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collimator with the glowlamp is rotatable about a’horizontal axis. The beam

FIG. 2. ROTATION OF THE NERNSTGLOWER SO AS TO PROVE THE ANGLEOF INCIDENCE EQUAL TO THE ANGLE

OF REFLECTION.

DEMONSTRATIONS IN LIGHT 1019

of light is visible across a vertical piece of ground glass.The equality of the two angles is at once apparent. It canalso be shown with this apparatus that a change of x° inthe angle of incidence changes the direction of the beamsby an angle 2x°; this fact is applicable to a rotatingmirror.

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FIG. 3. ARRANGEMENT OF MIRRORS SO AS TO SHOWREFRACTION. ROTATION OF THE MIRROR A WILL GIVE

TOTAL INTERNAL REFLECTION.

In Figure 3, a beam of light falls upon mirrors A andB and from them is directed into a tank of water tingedwith fluorescen. In the bottom of the tank is a thirdmirror C. When chalk dust is scattered along the lightbeam, the class can see the reflected rays from the threemirrors and the refraction of the rays at the surface ofthe fluid. If now the mirror A is rotated so that the lightray enters the side of the tank and goes in the fluid frombelow, the phenomenon of total internal reflection may bedemonstrated.

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FIG. 4. AN ARRANGEMENT OF LENSES WITH CONCAVE MIRROR TO SHOWTHE PROJECTION OF A REAL IMAGE UPON A SCREEN.

Figure 4 sliows an arrangement of lenses and mirrorsin which the rays are visible and the action of differentlenses may be studied. A pencil placed at the point A wi.llhave an image at the point B on the wall,