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



with an exceedingly small portion of nitric aether. This 

 causes in a short time a deposition of minute crystals in the 

 form of rhomboids, exquisitely perfect in shape and transpa- 

 rency, and of every variety of size. 



When these miniature crystals are placed on the stage of 

 the microscope, the field of view remaining dark as before, 

 we see a most interesting phenomenon ; for as every crystal 

 differs from the rest in thickness, it displays in consequence a 

 different tint, and the field of view appears scattered with the 

 most brilliant assemblage of rubies, topazes, emeralds, and 

 other highly coloured gems, affording one of the most pleas- 

 ing sights that can be imagined. The darkness of the ground 

 upon which they display themselves greatly enhances the 

 effect. Each crystal is uniform in colour over all its surface, 

 but if the plate of glass upon which they lie is turned round 

 in its own plane, the colour of each crystal is seen to change 

 and gradually to assume the complementary tint. Many 

 other salts may be substituted for the sulphate of copper in 

 this experiment, and each of them offers some peculiarity, 

 worthy of attention, but difficult to describe. Some salts, 

 however, crystallize in such thin plates that they have not 

 sufficient depolarizing power to become visible upon the dark 

 ground of the microscope. For instance, the little crystals of 

 sulphate of potash, precipitated by aether, appear only faintly 

 visible. In these circumstances a contrivance may be em- 

 ployed to render evident their action upon light. It must be 

 obvious that if a thin uniform plate of mica is viewed with the 

 microscope, it will appear coloured (the tint depending on the 

 thickness it may happen to have), and its appearance will be 

 everywhere alike, in other words it will produce a coloured 

 Jield of view. Now if such a plate of mica is laid beneath the 

 crystals, or beneath the glass which supports them, these 

 crystals, although incapable of producing any colour them- 

 selves, are yet frequently able to alter the colour which the 

 mica produces ; for instance, if the mica has produced a blue, 

 they will, perhaps, alter it to purple, and thus will have the 

 appearance of purple crystals lying on a blue ground. 



Such are the appearances when we examine crystals perfect 

 in form and structure. But the opposite extreme of the phae- 

 nomenon is also worth considering, namely, when the crystal- 

 lization is as irregular as possible, which is the case, for in- 

 stance, when a few drops of a saline solution are evaporated 

 by rapid heat. The salt chosen should of course be of a kind 

 which acts powerfully upon light. When different portions 

 of such a crystalline film are examined successively, the alter- 

 nations of colour succeed each other so rapidly and in such a 



