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VII. JS^ofe on a Citrioas Effect of Absorption of Light. 

 Bji iSiLVANus p. Thompson *. 



WHEN exi)eriinentino' upon the chromatic aljcrration of 

 the eve, I had occasion, as mentioned in a recent 

 communication to tlie Physical Society, to employ aqueous 

 solutions of porinanoanate of potash of various degrees of den- 

 sity as absorbing media. It is well known that solutions of 

 this salt, when sufficiently dense, absorb all the rays of the 

 middle portion of the spectrum, while freely transmitting red, 

 blue, and violet rays. Desiring, for the purposes of experi- 

 ment, to obtain a medium which should transmit in equal in- 

 tensity the two groups of rays of greatest and least refrangi- 

 bility, I found it advisable to somewhat reduce the intensity 

 of the red ra^'s bv passino- the beam of lio'ht throuoh a thin 

 glass plate tinted with pale cobalt. This plate was placed in 

 front of the glass trough containing the permanganate solu- 

 tion. The compound medium was totally opaque to rays be- 

 tween lines D and E, nearly opaque to rays between C and 

 I) in the orange and between E and F in the blue. 



The surfaces of objects reflecting ordinary white light, when 

 viewed through this medium, possessed a remarkable shinnner 

 or lustre (almost like the '' stereoscopic lustre " of Dove), as if 

 there were two surfaces both reflectino- lioht — a red surface, 

 and a blue surface behind it and seen through the red surface. 



When a coloured tablecloth or carpet was viewed through 

 the medium, the portions coloured yellow, orange, or red ap- 

 peared all equally red, and to be standing up above the surface 

 in a plane nearer to the eye. The otlier portions (those of blue 

 or green tint) appeared all alike of a blue-violet and as if sunk 

 to a lower plane. 



On observing a landscape-painting a similar effect was })ro- 

 duced, the blues and greys of the picture appearing of a blue- 

 violet, while all the yellows, reds, and even the bright greens 

 of the picture stood out, in deep-red tints, almost detached 

 from the plane of the picture. 



On turning from coloured drawings to an actual spring- 

 landscape the same startling phenomenon was repeated ; for, 

 while the sky gleamed with a steel-grey tint, the colour of the 

 young green leaves, the sunlit grass, and the tiled house-roofs 

 was alike Mood-red. It was hard to resist the notion that the 

 absorbent medium had actually changed the tint of the light ; 

 but actual comparison with the spectroscope showed that the 

 deep red observed existed among the rays of ordinary light 

 reflected by the objects in question. 



* Conimimicated bv the Author. 



