HEMOGLOBIN CEYSTALS OF EODENTS' BLOOD. 185 



joining a c, b d being the axes. Then if the angles dab, 

 d c b be replaced, as shown by the dotted lines, a hexagon 

 will be produced differing but little from a regular hexagon. 



3. The third alternative is that they may belong to the 

 rhombic system by being twins, consisting of three parallelo- 

 grams or six triangles, as is shown in figs. 2 and 3. Twins 

 are, however, rare in the rhombic system. 



In order to settle this question it is necessary to examine 

 the optical properties of the crystals. 



Crystals may be divided, according to their optical proper- 

 ties, into three classes : 



1. Isotropic. — Those in which there is no distinction of 

 different directions as regards optical properties. This includes 

 crystals belonging to the regular system. They have but 

 one refractive index, i. e. refract light like amorphous bodies 

 do, singly. 



2. Uniaxal. — Those in which the optical properties are 

 the same for all directions equally inclined to one particular 

 direction, called the optic axis, but vary according to this in- 

 clination. This class includes crystals belonging to the 

 dime trie system (crystals with three rectangular axes, two of 

 them being equal) and the hexagonal system. The optic 

 axis corresponds with the principal crystallographic axis. 

 In the direction of this axis a ray of light is refracted singly, 

 and in other directions doubly. 



3. Biaxal. — This includes the remaining three systems 

 of crystals, the trimetric or rhombic (three rectangular axes 

 all unequal), the monoclinic, and the trichinic. In these 

 there are always two directions along which a ray is singly 

 refracted. 



The best test, as to whether a substance is doubly refractive 

 or not, is this : If between crossed nicols, which consequently 

 appear dark, a substance be interposed that makes the dark- 

 ness give place to illumination, however feeble, that substance 

 is doubly refractive. This action is termed the depolarisation 

 of the ray. 



The crystals of squirrel's haemoglobin I submitted to this 



