On the Light received from the Heavenly Bodies. 21 



which are destitute of this property are much more numerous 

 than those which possess it. 



I believe, that as a distinctive character this optical pro- 

 perty may be of great service in chemistry. For instance, 

 suppose that an exceedingly minute fragment of crystal (con- 

 sidered to be nitre) were under examination. Let it be viewed 

 by polarized light in a manner analogous to what is above de- 

 scribed; then if, when turned in various positions, it fails to 

 develope two opposite colours, it follows that it cannot be a 

 particle of nitre. I conceive tljat this mode of examination 

 (when properly limited by experience) will prove a valuable 

 auxiliary to the others already known. 



Plate III. exhibits the phagnomena produced by the capil- 

 lary crystals of another salt — the oxalate of potash and chro- 

 mium. The two figures represent the same object: the change 

 of colour is effected by reversing the polarization of the light. 

 It will be understood that the same change takes place by 

 turning the object round 90°. From which it will be readily 

 seen to follow, that if a complete circle were formed of these 

 capillary crystals (all radiating from a point) two opposite 

 quadrants of it would appear green, and the other two red. 

 The crystallization of this salt is remarkably elegant, but I 

 have been obliged to confine myself in the plate to the repre- 

 sentation of one of its simplest forms, owing to the difficulty 

 of doing justice to those which are more complicated. I have 

 described it elsewhere*. 



Concerning the theory of these very pretty phfenomena, 

 I need say nothing in the present place, because I think that 

 I have given a satisfactory explanation, in the Philosophical 

 Transactions, of the cause from whence they originate. 



IV. On certain Conditions under •which Light is received 

 from the Heavenly Bodies^ and on the l7nj)orta7ice of investi- 

 gating them. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



I PRESUME upon your readiness to afford every facility 

 to suggestions on scientific subjects, in sending you the 

 few following remarks relating to a point connected with 

 optical philosophy, which seems to deserve some considera- 

 tion. Whether it really do contain anything worth further 

 inquiry will be best ascertained by publicity among your 

 readers. 

 * Fhii. Trans. 1837, I). -'33. [Sec L. & E. Phil. Mag., vol. x. p. 21J).J 



