46 



POLARISATION OF LIGHT. 



takes place in common polarisation, 

 when a sphere of glass has its density re- 

 gularly increasing or regularly diminish- 

 ing towards its centre. 



CHAPTER XIII. 



Absorption of Light in Doubly Refract- 

 ing Crystals List of absorbing 

 Crystals with one Axis List of ab- 

 sorbing Crystals with two Axes Pro- 

 perties of Dichroite Influence of 

 Heat in modifying the Absorption of 

 Light by Crystals. 



EVERY substance, however transparent, 

 possesses the property of absorbing a 

 portion of the light which falls upon it* ; 

 and the quantity of light absorbed by 

 homogeneous fluids and solids that have 

 single" refraction is the same, whatever 

 be the direction in which the ray is 

 transmitted. In doubly refracting crys- 

 tals, however, the absorption is of a dif- 

 ferent kind. They not only possess the 

 power of absorbing light like other 

 bodies, but they possess another absorp- 

 tive force which is related to the axis of 

 the crystal, and intimately connected 

 with its doubly refracting and polarising 

 forces. In order to analyse this pheno- 

 menon, Dr. Brewster, who discovered this 

 property in crystals, took a rhomb of 

 yellow carbonate of lime, of such a 



thickness as to give two distinct and 

 separate images of a small circular aper- 

 ture placed before it and illuminated with 

 white light. In this case, he found that 

 the two images differed both in colour 

 and intensity, the extraordinary image 

 having an orange yellow hue, while the 

 colour of the ordinary image was a yel- 

 lowish white. Along the axis, the two 

 images had the same colour and inten- 

 sity ; at different inclinations to the axis, 

 the difference in the colour and intensity 

 of the two pencils increased, and was a 

 maximum at 90. When the two 

 images were made to overlap at dif- 

 ferent inclinations with the axis, their 

 combined colour was always the same, 

 and was of course the same with the 

 natural colour of the mineral, which ap- 

 peared to be the same in all direc- 

 tions. When the rhomb is exposed to 

 polarised light, the following effects are 

 produced. In the position where the ordi- 

 nary image O vanishes, the extraordi- 

 nary image E is orange yellow, exactly 

 the same as it appeared by common 

 light ; and in the position where E 

 vanishes, O is a yellowish white as before. 

 The property now described was 

 found by Dr. Brewster in the following 

 crystals with one axis, in most of which 

 the two images O and E exhibited two 

 different colours. 



List of Crystals with one Axis that absorb 



Colour when its principal section is in 

 the plane of primitive Polarisation. 



. Brownish white . 

 Yellowish green 



. Pale yellow 

 Yellowish green 



. Bluish green . 

 Bluish white 



. Whitish 

 Pale yellow 

 Whitish 



Yellowish white 

 Blue 



Greyish white . 

 Reddish yellow 

 Greenish white 

 Reddish white 

 Yellow . 

 Yellow . . 



Bluish . 

 Bluish green 

 Bright green 

 Orange yellow 

 Whitish brown 



Names of Crystals. 

 Zircon 



Sapphire .... 

 Ruby .... 

 Emerald .... 

 Emerald 

 Beryl Blue 

 Beryl green . 

 Beryl yellow green 

 Rock crystal almost transparent 

 Rock crystal yellow 

 Amethyst .... 

 Amethyst 

 Amethyst . 

 Tourmaline . 



Rubellite .... 

 Idocrase 



Mellite . . . 



Apatite, lilac . . . 



Apatite, olive . . 

 Phosphate of lead . 

 Calcareous spar . 

 Octohednte . 



different Colours. 



Colour when its principal section 

 is perpendicular to that plane. 

 . A deeper brown 



Blue 

 . Bright pink 



Blui-h green 

 , . Yellowish green 



Blue 

 . Bluish green 



Pale green 

 . Faint brown 



Yellow 

 . Pink 



Ruby red 

 . Kuby red 



Bluish green 

 . Faint red 



Green 

 . Bluish white 



Reddish 

 . Yellowish green 



Orange yellow 

 . Yellowish white 



Yellowish brown 



The absorptive property is not pos- 

 sessed by every specimen of the minerals 



* See Optics, chap. xix. p. 6?. 



contained in the preceding list. Even 

 when they have the same colour as those 

 above described, the ordinary and ex- 

 traordinary pencils which they produce 



