THE BLOOD. 



Fig. 54. 



of an inch, or a little less than one-fifth of its transverse 

 diameter. 



When the globules are examined lying upon their broad sur- 

 faces, it can be seen that these surfaces are not exactly flat, but that 



there is on each side a slight 

 central depression, so that 

 the rounded edges of the 

 blood-globule are evidently 

 thicker than its middle por- 

 tion. This inequality pro- 

 duces a remarkable optical 

 effect. The substance of 

 which the blood-globule is 

 composed refracts light more 

 strongly than the fluid plas- 

 ma. Therefore, when exa- 

 mined with the microscope, 

 by transmitted light, the 



HUMAN BLOOD-GLOBULES. -. Red globules, thick edges of the globules 

 seen flatwise, b. Red globules, seen edgewise, c. ac {; as double COnVCX lenses, 

 White globule. 



and concentrate the light 



above the level of the fluid. Consequently, if the object-glass be 

 carried upward by the adjusting screw of the microscope, and lifted 



away from the stage, so that 

 the blood-globules fall be- 

 yond its focus, their edges 

 will appear brighter. But 

 the central portion of each 

 globule, being excavated on 

 both sides, acts as a double 

 concave lens, and disperses 

 the light from a point below 

 the level of the fluid. It, 

 therefore, grows brighter as 

 the object-glass is carried 

 downward, and the object 

 falls within its focus. An 

 alternating appearance of the 

 blood-globules may, there- 

 fore, be produced by view- 

 ing them first beyond and then within the focus of the instrument. 



Fig. 55. 



RED GLOBULES OF THE BLOOD, seen a 



beyond the focus of the microscope. 



little 



