24. _Search-Light._--(Fig. 102). This is simply an arc lamp with reflectors behind it and lenses in front of it. The whole apparatus is pivoted so as to be easily made to shine in any direction. The function of the lenses and the reflectors is to collect stray rays of light and send them all out in the same direction. This is shown in Fig. 103 where for simplicity the lens is represented as a single piece. _L_ represents a point of light which will naturally send its rays out in all directions as the radii of a sphere; _m_, _m_, _m_ represents a bright reflecting surface which is given that peculiar curve called a parabola. It has the unique faculty of reflecting in a parallel direction all the rays which may fall upon it from _L_, so long as _L_ is kept at that particular point called the focus, _a b_ is a lens of glass which has that peculiar curve that enables it to bend all rays which fall upon it from _L_, so that they may pass out parallel. [Illustration: Fig. 102] 25. _Stereopticon._--This also has the necessary devices to gather the rays of the arc lamp and send them forth parallel, and in addition it has a series of lenses which produce upon a distant screen an enlarged picture of any transparent object held in these parallel rays. [Illustration: Fig. 103] [Illustration: Fig. 104] 26. _Burglar's Flash-Light._--There are many forms of this. The one we examined is represented in Fig. 104. We unscrewed a metal ring at the left-hand end and found, first a glass lens and behind that a miniature electric light, requiring three volts and half an ampere. We knew, therefore, that it must be supplied with two cells, since one cell may give not more than 1.5 volts. We also knew that it would only be used to _flash_ a light, since if dry cells are required to furnish half an ampere continuously they soon run down. Behind the lamp there was a bright metal reflector--the lens and reflector are fairly well represented in Fig. 103. The filament of the lamp is connected with two small battery cells in the handle. These may be removed and replaced by new ones by unscrewing a cap at the right-hand end. The circuit is closed by a metal spring on the side of the tube, which acts as a push button. It is situated where it may be conveniently pressed by the thumb. The small batteries necessarily have a short life and must be replaced quite frequently. Being a special thing they cost nearly twice what the regular dry cell does. [Illustration: Fig. 105]