BLOOD. 



45- 



thin layer it, too, is yellow. In a cubic millimeter (^ inch) 

 of blood there are about five million red corpuscles. Struc- 

 ture. Seen from the front the central part of each red cor- 

 puscle in a certain focus of the microscope appears dimmer or 

 darker than the rest (Fig. 10, B), except a narrow band 

 near the outer rim. If the lens of the microscope be raised, 

 however, this previously dimmer central part becomes 

 brighter, and the previously brighter part obscure (E).. 



FIG. JO. Blood corpuscles. A, magnified about 400 diameters. The red cor' 

 puscles have arranged themselves in rouleaux ; a, a, colorless corpuscles ; B^ 

 red corpuscles more magnified and seen in focus ; E, a red corpuscle slightly 

 out of focus. Near the right-hand top corner is a red corpuscle seen in three- 

 quarter face, and at C one seen edgewise. F, G, H, I, white corpuscles highly- 

 magnified. 



This difference in appearance does not indicate the presence- 

 of a central part or nucleus different from the rest, but is 

 an optical phenomenon due to the shape of the corpuscle, 

 in consequence of which it acts like a little biconcave lens 

 (see Physics). Rays of light passing through near the 

 centre of the corpuscle are refracted differently from those 

 passing through elsewhere; and when the microscope is 

 so focused that the latter reach the eye, the former do not A 



