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XLIII. On the Existence of the Colour Brown. 

 By Ernst Brucke*. 



DROWN is wanting in the prismatic spectrum, and its 

 relation to the colours of the spectrum is as yet unknown. 

 Any one may, however, easily convince himself that brown is 

 nothing more than the complementary colour to that of Her- 

 schel's lavender-gray rays, *. e. white light from which these 

 rays have been removed. 



For this purpose, separate plates should be split from cry- 

 stallized gypsum in such a manner that on one side they are 

 as thin as possible, and from it gradually increase in thickness 

 in broad terraces. One of these plates is placed under the 

 microscope, which must be furnished with two NichoPs 

 prisms, one beneath the object-glass, and one in the eye-piece, 

 and so arranged, the prisms being parallel, and the linear 

 magnifying power being about twenty diameters (at a di- 

 stance of eight French inches), that the above-mentioned thin 

 side is in the field. If it is sufficiently thin, no colour is per- 

 ceived immediately at the side ; but as we proceed towards 

 the thicker part, at first a pale brown tint becomes visible, as 

 if we were looking through a very thin plate of horn, and as 

 the thickness of the plate gradually increases in broad and 

 low terraces, the brown continues to become darker until it 

 assumes a deep and pure nut-brown colour, without the inter- 

 vention of any of the prismatic colours which the thicker parts 

 of the plate exhibit. 



It is evident that the plate at the margin where it appears 

 colourless is so thin, that the difference of the path of the 

 ordinary and extraordinary ray on their exit does not amount 

 to half the length of a wave for any colour. Thus interference 

 of the most refractive rays does not occur until the thickness 

 is greater, and the brown colour must therefore be produced 

 by the disappearance of the lavender-gray rays from the com- 

 pound light. 



The correctness of this conclusion is readily tested. On 

 crossing the prisms, it is seen that whilst in the case of all the 

 other colours of the plate the well-known complementary co- 

 lours appear, that portion which was previously brown becomes 

 coloured lavender-gray, and the intensity of this colour is in 

 proportion to the depth of the brown previously observed at 

 the same spot. 



Kdnigsberg, June 11, 1848. 



* From Poggendorff's Annalen, No. 7, 1848, having been read before 

 the Physical Society of Berlin on the 23rd of June 1848. 



Phil. Mag. S. 3. Vol. 33. No. 222. Oct. 1848. U 



