POLARISED LIGHT THROUGH DOUBLE REFRACTING CRYSTALS. 185 



coloured ring crosses a coloured ring, the intersection is of the resultant colour 

 of the two combined. 



Still more complex figures are got by employing two or more crystals in 

 combination. 



Indeed, there is no end to the variety of exquisite beauty, both in colour and 

 in pattern, which a little ingenuity may produce. Pigments would be almost 

 as helpless as words in representing many of these. The appearance produced 

 by a single crystal with a double image prism as analyser, may be not inaptly 

 compared to a tesselated pavement of every colour made for a fairy palace, 

 while that produced by combining two crystals may be said to resemble a suit 

 of chain armour wrought for a fairy king in jewels of which no two are of the 

 same hue. 



Addition to the above Paper. By J. Clerk Maxwell, LL.D., F.R.SS. L. & E. 



In Mr Deas' paper a number of interesting experiments are described, in 

 which, by means of a spectroscopic microscope fitted with polarising and analys- 

 ing prisms, the true nature of the phenomena observed by Brewster, Biot, 

 and others, in plates of selenite, &c, is made exceedingly intelligible to the 

 understanding, while, at the same time, the eye is satiated with new forms of 

 splendour. 



The subject is one to which the attention of experimenters is not so strongly 

 directed as it was fifty years ago ; and therefore it is desirable that the remark- 

 ably simple methods of observation here described, and the perfection with which 

 the phenomena may be seen by means of modern instruments, should be more 

 generally known. 



In the text, the paper appears purely descriptive, without any theoretical 

 application, and the aesthetic beauty of the phenomena might be assumed to be 

 the object of the experiments. But the carefulness of the selection of the 

 experiments and the faithfulness of the description make me think that the 

 author himself looked at what he saw in the light of the theory of double 

 refraction and the interference of light. I, therefore, think that a simple state- 

 ment of the relation of the visible things here described to the results of theory 

 would greatly increase the value of the paper ; for in scientific education the 

 identification of what is observed with what is deduced from theory is of more 

 value than either the process of observation or the process of deduction. 



This might be done as follows : — 



Begin with the plane polarised light, the equations of motion of which are 



x = c cos nt y — 0. 

 Now let it pass through a plate of crystal of which the axis is inclined a to 



VOL. XXVI. PART I. 3C 



