Mr. H. F. Talbot's Experiments on Light. 325 



capricious manner, as to baffle description, and with care it is 

 often possible to detect five or six different shades of colour 

 upon some of the very smallest of the crystalline fragments. 

 These miniature phenomena are often highly curious. Upon 

 one occasion a solution of some salt was precipitated by al- 

 cohol in very minute crystals. When the alcohol began to 

 evaporate, it caused currents in the liquid, which carried the 

 crystals across the field of view, and at the same time caused 

 them to revolve round their centres. 



The crystals, which were highly luminous in one position, 

 when their axes were in the proper direction for depolarizing 

 the light, became entirely dark in the opposite position, thus, 

 as they rapidly moved onwards, appearing by turns luminous 

 and obscure, and resembling in miniature the coruscations of 

 a firefly. It was impossible to view this without admiring the 

 infinite perfection of nature, that such almost imperceptible 

 atoms should be found to have a regular structure capable of 

 acting upon light in the same manner as the largest masses, 

 and that the element of light itself should obey in such trivial 

 particulars the same laws which regulate its course through- 

 out the universe. 



In what has preceded I have always supposed the polarizing 

 eyepiece in such a situation as to darken the field of view. If 

 we now turn it round 90°, the field of view becomes bright. 

 But we are now presented with a new set of appearances, 

 which though not so striking as the last, still have consider- 

 able interest. Even in this position the apparatus often im- 

 parts a strong tinge of colour to crystals which are quite 

 white by ordinary light; and this is sometimes so intense as 

 to produce complete opacity. As this fact appears to me to 

 be new, and is particularly striking, I will mention that I have 

 most frequently observed it with the salts of copper and nickel, 

 but it also occurs with other salts. 



The singularity of this appearance is enhanced by turning 

 round the polarizing eyepiece ; for when the field of view be- 

 comes dark, the crystal becomes white and luminous, and 

 when the field becomes bright, the crystal darkens and becomes 

 entirely opaque. I have not yet satisfied myself concerning 

 the cause of this remarkable phaenomenon, which is not in 

 accordance with the received theory ; but it probably arises 

 from something peculiar in the chemical nature of these cry- 

 stals, which causes them to deviate from the ordinary law of 

 action with respect to the differently coloured rays. 



A circumstance which frequently accompanies these experi- 

 ments is too remarkable to be passed over in silence. 



The crystals which form in the liquid under examination are 



