10 EIGHTH REPORT — 1838. 



On a New Phenomenon of Colour in certain specimens of Fluor Spar. 

 By Sir D. Brewster. 



Mineralogists have long ago observed, in certain varieties of fluor 

 spar, a beautiful blue colour, different from that which is seen by trans- 

 mitted light. Haiiy noticed this property in some of the fluor spars 

 from Derbyshire. Succeeding mineralogists, however, have confounded 

 this colour with the ordinary tints of the spar, and, so far as the author 

 knows, its nature and origin have not been successfully investigated. In 

 describing a species of dichroism, noticed by Dr. Prout * in the purpu- 

 rates of ammonia and potash. Sir John Herschel f ascribes the reflected 

 green light to " some peculiar conformation of the green surfaces, pro- 

 ducing what may be best termed a superficial colour, or one analogous 

 to the colour of thin plates, and striated or dotted surfaces." And he 

 adds — " A remarkable example of such superficial colour, differing 

 from the transmitted tints, is met with in the green fluor of Alston 

 Moor, which on its surfaces, whether natural or artificial, exhibits, in 

 certain lights, a deep blue tint, not to be removed by any polishing." 

 As the phenomenon which Sir D. Brewster had studied in the Derby- 

 shire fluors was clearly one of internal structure, he was led to sup- 

 pose that the superficial colour seen by Sir John Herschel on the 

 Alston Moor specimens, belonged to another class of phenomena; but 

 having attempted in vain to communicate the blue colour of the Alston 

 Moor crystals to wax or isinglass, he is disposed to believe that the 

 two phenomena are identical. In the fluors from Derbyshire, which 

 consist of differently coloured strata parallel to the faces of the cube, 

 the blue colour is most powerfully developed in the purplish brown or 

 bluish brown strata, in a less degree in the greenish strata, and scarcely, 

 if at all, in those layers which are colourless by transmitted light. In 

 the first of these cases, the blue colour may be distinctly seen emanating 

 from the interior of the crystal, when it is held in the common light of 

 day. In the sun's light the colour is still more brilliant; but the effect 

 may be greatly increased by covering the greater part of the crystal 

 with black wax, or by immersing it in a trough of glass covered ex- 

 ternally with wax, and containing an oil of nearly the same refractive 

 power as the spar. If there are fissures within the crj stal, they M'ill 

 greatly influence the effect of the experiment, by reflecting to the eye 

 the transmitted light. In order, however, to witness this experiment 

 in all its beauty, and to have ocular evidence of its nature and charac- 

 ter, a beam of condensed solar light should be transmitted through the 

 crystal, as shown in the annexed figure, where l l is the condensing 

 lens, F its focus, and m n the system of diffierently coloured layers, tra- 

 versed by the cone of refracted rays. The first layer of spar reflects 

 in all directions an intensely blue light; the two adjacent layers (sepa- 

 rated by a thin layer which reflects blue light) reflect a light nearly 

 white ; the next layer gives a blue of exceeding brilliancy ; and so on 

 with the other layers, till the cone reaches tiie brown central nucleus, 

 which also reflects a rich blue tint, though inferior to that of one of 



• Phil. Trans. ISIS, p. 424. t Treatise on Light, sec. 1076. 



