186 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



then placed by the side of the standard gradations, and the eye quickly 

 recognises its approximate position on the scale. The camera tube provided 

 with the instrument will more accurately define it. Artificial light should 

 be used. 



If it is proved that the blood solution is matched in depth of colour by 

 one of the standard grades, the observation is at- an end ; but if the tint is 

 higher than one grade, but lower than another, the blood cell is placed 

 opposite to the former, and riders (not shown in the illustration) are added 

 to complete the observation. The standard gradations are marked in per- 

 centages, 100 per cent, being taken as normal. 



' Worth ' of the Corpuscles. — If the percentage of hsemoglobia is 100, and the 



100 

 percentage number of corpuscles is 100 also, then the quotient — — = 1 is 



taken as the normal. This varies in health from 0'95 to 1*05 in men, and 

 from 0*9 to 1 in women. This quotient has been termed the worth of the 

 corpuscle. 



Specific Gravity of Blood. — Of the numerous methods introduced for taking 

 the specific gravity of fresh blood, that of Hammerschlag is the simplest. A 

 drop of blood from the finger is placed in a mixture of chloroform and 

 benzene. If the drop falls, add chloroform till it just begins to rise ; if the drop 

 rises, add benzene till it just begins to fall. The fluid will then be of the 

 same specific gravity as the blood. Take the specific gravity of the mixture 

 in the usual way with an accurate hydrometer. 



Schmalz's capillary picnometer is more accurate. 



POLARISATION OF LIGHT 



If an object, such as a black dot on a piece of white paper, be looked at 

 through a crystal of Iceland spar, two black dots will be seen ; and if the 

 crystal be rotated, one black dot will move round the other, which remains 

 stationary. That is, rays of light entering such a crystal are spht into two 

 rays, which travel through the crystal with different velocities, and conse- 

 quently one is more refracted than the other. One ray travels just as it 

 would through glass ; this is the ordinary ray, the ray which gives the 

 stationary image ; the other ray gives the moveable image when the crystal 

 is rotated ; the ordinary laws of refraction do not apply to it, and it is called 

 the extraordinary ray. Both rays are of equal brilliancy. In one direction, 

 however, that of the optic axis of the crystal, a ray of light is transmitted 

 without double refraction. 



Ordinary light, according to the wave theory, is due to vibrations occur- 

 ring in all planes transversely to the direction of the propagation of the wave. 

 Light is said to be plane polarised when the vibrations take place all in one 

 plane. The two rays produced by double refraction are both polarised, one 

 in one plane, the other in a plane at right angles to this one. Doubly refract- 

 ing bodies are called anisotropous ; singly refracting bodies, isotropous. 

 The effect of polarisation may be very roughly illustrated by a model. 



If a string be stretched as in the figure, and then touched with the finger, 

 it can be made to vibrate, and the vibrations will be free to occvir from above 

 down, or from side to side, or in any intermediate position. If, however, a 



