Photographing witli Monochromatic Rays. 13 



As a verification of this theory, I beg leave to project on the 

 screen a series of colour photographs, representing natural objects : 

 pictures on stained glass, landscapes from nature, flowers, and a 

 portrait from life. Every colour in nature, including white, and the 

 delicate hue of the human 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 pigments, these would be made apparent by slightly turning 

 the photograph ; these pigments remaining visible under every in- 

 cidence, they would thus be seen to stand out on a colourless back- 

 ground. Thus the touching up or falsifying by hand of a colour 

 photograph is happily made impossible. 



"Note on Photographing Sources of Light with Mono- 

 chromatic Rays." By Captain W. DE W. ABNEY, C.B., 

 D.C.L., F.R.S. Received March 31, Read April 30, 1896. 



In a paper " On the Production of Monochromatic Light," com- 

 municated 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 that by the 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 monochromatic image of an external 

 object, in which a concave lens was placed behind the slit of a spectro- 

 scope to produce an image of the object in monochromatic colour, 

 the object being viewed through an aperture placed in the spectrum 

 produced by the apparatus. I had been working independently at 

 the subject at the same time, and my object was to get an image on a 

 screen or photographic plate rather than to use the apparatus for 

 visual observation. When a lens is placed behind the spectrum in 

 the manner described in the paper above referred to, a white 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 diminished 

 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 luminous 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 



