Mr. H. F. Talbot's Experiments on Light. 323 



it perfectly transparent, and to prevent the diffraction of light, 

 it should be immersed in oil or varnish and pressed between 

 two plates of glass. The field of view having been darkened 

 as much as possible in the manner before described, the hair 

 is placed upon the stage of the microscope, and it is then seen 

 to be beautifully luminous. As no other light reaches the 

 eye than that which traverses the hair, every minute circum- 

 stance in its interior structure is plainly distinguished. Jt is 

 almost needless to say that this effect is owing to the de- 

 polarizing power of the hair. Many organic substances of 

 animal and vegetable origin appear luminous in a similar way, 

 while others, on the contrary, are inert, and have no such ac- 

 tion upon light. 



Fragments of coarsely powdered sugar and of various kinds 

 of salts appear more or less bright, and mottled with various 

 colours; but common salt is an exception, as it remains nearly 

 or altogether dark, at least when it is employed in a state of 

 purity. 



But when instead of examining amorphous fragments of 

 these salts we view them regularly and carefully crystallized, 

 the phaenomena become much more interesting. Sulphate of 

 copper offers a very excellent example. This salt, which is of 

 a fine blue colour when viewed in considerable thicknesses, 

 is white and transparent when it is extremely thin, and its 

 crystals can be procured so small as to be quite destitute of 

 perceptible coloration. A drop of it was placed upon a warm 

 piece of glass and suffered to evaporate gradually. The cry- 

 stals shooting from the edge of the drop into the interior of 

 the liquid had a long and narrow rectangular form, with a 

 slanting extremity, which may be compared in shape to the 

 straight edge of a chisel. Seen by common light these cry- 

 stals offer nothing peculiar, but on the darkened field of the 

 microscope they are luminous and splendidly coloured, the 

 colour depending upon the thickness of the crystal, and being 

 the same in all points of its surface, except upon the little in- 

 clined plane which forms its extremity. But upon this oblique 

 portion are seen three or four distinct bands of colour parallel 

 to the edge and offering to the eye a visible scale or measure 

 of the rapid diminution of thickness in that part. The ob- 

 served succession of colours in one experiment was the fol- 

 lowing. Yellow, brown, purple, blue, sky blue, straw yellow, 

 yellow, reddish purple, blue, sea green, green, greenish yellow, 

 pink, green, pink, blueish green, pink. 



Taking, again, for the subject of experiment the sulphate of 

 copper, we may make it crystallize in quite another manner, 

 by not employing any heat, but simply touching the drop 



2T2 



