Pure water certainly absorbs the -infra-red rays of great wavelength, but not those near to the visible extreme red, and both kinds produce heat only. The absorption of the infra red can be increased only by the addition to the water of a substance of selective absorption which also absorbs some of the useful light. It is obviously necessary to select a salt of which a solution of useful strength does not transmit light of a very marked colour. For instance, in troughs of about 2 in. thickness a io per cent solution of ferrous sulphate may be used (C. E. K. Nees, 1912 ; this is very oxidizable in spite of the addition of a little sulphuric acid), or a 1•5 per cent solution of copper sulphate, which is perfectly stable, may be used (W. Crookes, 1921). It is best to use distilled water, or, failing this, water which has been boiled, to avoid the formation of lime deposits on the glass. The water trough may be connected to a tank at a higher level, so as to ensure a continuous circu lation, thereby avoiding overheating, with the consequent risk of bubbles. The water in the trough may be cooled by a coil through which cold water circulates.
It has been suggested that the light-source (incandescent lamps) be immersed in a water trough (E. Borlandi, 1912) or to combine two water troughs with a slide carrier (P. Feron Vrau, 1921) for use alternately.
In cinema projectors it has been common practice for some years past to cool the film by blowing moist air against both its surfaces.
Glasses able to withstand considerable heat ing, and slightly tinted a greenish blue, absorb the greater part of the infra-red rays and absorb only a little light. They can therefore be used with advantage either separately or as the sides of water troughs. They can themselves be cooled by having air blown on to them.
799. Projection of Opaque Objects. " Epi scopic " projection (by the light diffused by an opaque body) is of very low efficiency, since the lens collects only a very small fraction of the light scattered in all directions by the opaque object which it is desired to project. For this
reason projectors for this purpose did not make their appearance until very powerful sources of light became available (in this case the total luminous intensity, and not the brightness, must be considered).
Various patterns have been worked out, ranging from toys intended for family use with picture postcards to the large educational in struments which permit the projection of the pages of an atlas or of objects in low relief (medals, botanical specimens, small animals being dissected, etc.). Some of these instru ments (epidiascopes) serve both for episcopic projection and for transparency projection (diascopic projection).
The illumination is provided, direct or by reflection from mirrors, by an arc lamp or by powerful incandescent lamps. Condensers pre vent an excessive dispersion of the light, and adjustment is then made so as to direct one or more parallel beams on to the original to be projected.
In the toy patterns and in the simplified episcopic projectors, the original is introduced in vertical grooves, or is placed behind an aperture in a vertical panel, the lens then projecting on the screen a picture which is reversed as regards left and right.' This fact prevents the reading of titles, graduations of charts, reference letters, etc. Preferable to these are the instruments in which the original or objects to be projected are placed on a horizontal table. The optical axis of the lens is then ver tical, the emergent beam being then reflected in a horizontal direction by a reflector (mirror or prism) which turns the image the right way round and thereby ensures the picture being viewed correctly on an opaque screen.