C 2 7° 3 
XVIII. 0« the multiplication of images , and the colours which 
accompany them in some specimens of calcareous spar. By 
David Brewster, LL.D. F. R. S. Bond, and Edin. In a Letter 
addressed to the Right Hon. Sir Joseph Banks, Bart. G. C. B 
P.R.S. 
Read June 8, 1815. 
Dear Sir, 
The multiplication of images exhibited in some specimens 
of Iceland spar, appears to have been first observed by Dr. 
John Robison of Edinburgh, who showed the phenomenon 
to Mr. Benjamin Martin.* Having procured several spe- 
cimens that had a similar property, Mr. Martin examined 
them with care, and published an account of his observations 
in his Essay on Iceland Crystal. The experiments of Mar- 
tin were repeated by Mr. Brougham, who concluded that 
the images were produced by fractures, parallel or nearly so 
to the sides of the rhomboid, and Malus has more recently 
endeavoured to explain the phenomena, by the laws of extra- 
ordinary reflexion within doubly refracting crystals.-f* 
All these philosophers agree in ascribing the multiplication 
of images to internal reflections, and they equally concur in 
regarding the colours of the images as the same with those 
of thin plates, and as produced by fissures, or fractures within 
the crystal. 
* See Brougham’s “ Further Experiments, Sec. on Light.” Phil. Trans. 1 797. 
f I am acquainted with Malus’s observations only from a short account of 
them given by Dr. Young. 
