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DRY PLATES, FILMS AND PAPERS 161
back to the under surface of neighbouring parts of the film. As it is the photographically active rays which must be absorbed, the backing must be of a reddish brown or black colour. Again, the refractive index of the material used should be the same as that of the glass, if it is to have a maximum effect in preventing reflection of the light.
The advantage of using such backed plates is well illustrated in figs. 75 and 76.
Fig. 75 shows the halation effect due to the action of the excess of light which has passed through the window. The surrounding details are almost entirely obliterated. Not only that, but the brighter portion of the window has even produced a reversal effect — i.e. a positive instead of a negative has appeared on developing the plate. Fig. 76 shows the result obtained when an Ilf ord Rapid Chromatic Backed Plate is used for obtaining the same photograph. Here all the details of the window and its immediate neighbourhood are quite distinct.
Backing adds greatly to the effectiveness of plates when used in astronomical work. Any fine dark markings on a brightly illuminated area, e.g., the surface of the moon, tend to become so weakened in detail by halation as to be very indistinctly seen.
When a ray of light which has passed through the sensitive film and the glass plate at any angle other than the normal, meets the air, a portion of the ray is reflected again into the glass, and strikes against the lower surface of the film, as previously stated.
The amount of light so reflected gradually increases with the angle of incidence until the critical angle is reached, at which point there is total reflection. The point upon the film where the ray falls may be considered as the point of emission, and as the rays radiate in all directions around this point, they thus form a circle in the plane of the sensitive film.