The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Photography in Natural Colours
exposure - for instance, shortly before sunset - the violet, blue, and ultra-violet are not reproduced. The ultra-violet first dis-appears, then the blue, the place of the latter being taken up by a prolongation of the green, or shows a steely-grey colour. (6) With sufficiently long exposure the, to the eye invisible, infra-red (beyond the line A) appears as a dark purple, the ultra-violet (beyond H) sinking into yellowish rose-red lavender. (7) In the ultra-violet (beyond H) there appears, when an electric arc light is used, a very intense light maximum which is separated from the H group by a group of colourless carbon lines, this maximum being visually recognised, and with sufficient exposure, is of a deeper and more intense dark blue than the indigo blue of the solar spectrum. (8) The actinic in-
tensity of the electric arc light, at a distance of 36 cm. from the slit of the spectroscope to the positive pole of the carbons, is to direct sunlight (midday, April, clear sky) as 1 : 38 to 1 : 40. (9) Only by using Lippmann's mercury mirror, when the require-ments of rule 3 are complied with, is it possible to obtain the photographic colour results corresponding to the visual spectrum and the colours in their correct places. Reflected light
acts alwaysjmore strongly than direct
Fig. 105.
light, and Krone recommended in the first place the use of a metallic mirror in order to avoid the for-mation of two spectrum images which would give rise to the formation of mixed colours in the pictures. Fig. 104 shows Lippmann's arrangement for obtaining a spectrum. L is an electric arc lamp, the rays from which are condensed by a lens upon f which is an opaque screen with a narrow slit in it. a is a convergent lens which renders the rays parallel from whence they strike the prisms P, and are formed into an image by o, the lens of the camera c, on E, the dark slide of particular form. To view these photographs in natural colours it is necessary to cement them to a prism of small angle and back them with black velvet or black varnish. The method of showing them by artificial light being sketched in fig. 105, in which a is the lantern, c the