Page:Proceedings of the Royal Society of London Vol 60.djvu/21

Rh As a verification of th is theory, I beg leave to project on the screen a series of colour photographs, representing n atu ral objects : pictures on stained glass, landscapes from nature, flowers, and a p o rtrait from life. E very colour in nature, including w hite, and th e delicate hue of th e hum an complexion, is thus shown to be reflected by a correctly developed photographic film.

It is to be remarked that, as in the case of the spectrum, the colours are visible only in the direction of specular reflection. If I had tried to touch up these photographs by means of water colours or other pigm ents, these would be m ade apparent by slightly turning the p hotograph; these pigm ents rem aining visible und er every in ­ cidence, they would th u s be seen to stand out on a colourless background. Thus th e touching up or falsifying by h and of a colour photograph is happily m ade impossible.

In a paper “ On the Production of M onochromatic L ig h t,” comm unicated to the Physical Society, and read on the 27th June, 1885, and which appears in the ‘ Philosophical Magazine’ for August in that same year, I stated th a t by th e apparatus then described a monochromatic image of the sun could be thrown upon the screen. In the same periodical for June of the same year, Lord Rayleigh described a plan for obtaining a monochrom atic image of an external object, in which a concave lens was placed behind the slit of a spectroscope to produce an im age of the object in monochromatic colour, the object being viewed through an aperture placed in the spectrum produced by the apparatus. I had been w orking independently at th e subject at the same tim e, and my object was to get an im age on a screen or photographic plate rath er th an to use the apparatus for visual observation. W hen a lens is placed behind the spectrum in the m anner described in the paper above referred to, a w hite image of the prism can be obtained on a screen placed at some distance from the lens, and the size of the image can be increased or dim inished according to the focal length of the lens, and its distance from the spectrum. Evidently, then, if an image of a luminous object can be cast on the surface of the prism, and a slit be placed in the spectrum, the image of the lum inous object will be seen of the colour of the light passing through the slit. There are devices adopted at the present time for photographing the sun with light of various wave-lengths, but, as far as I am aware, they depend upon moving the image of the sun across the slit of the spectroscope, the