120 



THE MICKOSCOPE. 



fig. 86. Snow Crystals. 



and accurately recorded, would be an interesting feature in meteoro- 

 logical investigation. At the same time, it is probable that the con- 

 ditions of their forma- 

 tion are more complex 

 than might be imagined, 

 familiar as we are with 

 the conditions relating 

 to the crystallisation of 

 water on the earth's 

 surface. Dr. Smallwood, 

 of Isle Jesus, Canada 

 East, has traced an ap- 

 parent connection be- 

 tween the form of the 

 compound varieties of 

 snow crystals and the 

 electrical condition of 

 the atmosphere, whe- 

 ther negative or positive; and is, he informs me, instituting experi- 

 ments for his better information on the subject." 



A great variety of animal, vegetable, and other substances possess 

 a doubly refracting or depolarising structure, as : a quill cut and laid 

 out flat on glass ; the cornea of a sheep's eye ; skin, hair, a thin sec- 

 tion of a finger-nail ; sections of bone, teeth, horn, silk, cotton, whale- 

 bone ; stems of plants containing silica or flint ; barley, wheat, &c. 

 The larger-grained starches form splendid objects ; tons les niois, being 

 the largest, may be taken as the type of all the others. It presents a 

 black cross, as at a, fig. 87, the arms of which meet at ttie hilum. On 



rotating the analyser, the black cross 

 disappears, and at 90 is replaced by a 

 white cross, as at b ; another, but much 

 fainter black cross being perceived 

 between the arms of the white cross. 

 Hitherto, however, no colour is per- 

 ceptible. But if a thin plate of sele- 

 nite be interposed between the starch- 

 fi 7 grains and the polariser, most splendid 



and delicate colours appear. All the 



colours change by revolving the analyser, and become complementary 

 at every quadrature of the circle. West and East India arrow-root, 

 sago, tapioca, and many other starch-grains, present the same appear- 



