28 PRACTICAL HISTOLOGY. 



wards; so that if the objective is at a certain distance, 

 all of the rays which traverse the central part do not 

 reach it, some of them being deflected too much to the 

 side to impinge upon the lower glass of the objective. 

 The part in question, therefore, looks a little dimmer than 

 the somewhat convex marginal part, whereas when the 

 objective is brought nearer all the rays are intercepted 

 by it; and the middle part. owing to its greater thinness, 

 appears lighter than the rim. If a very high power ob- 

 jective is used, the middle part of the corpuscle will be the 

 lighter, even when the focus is rightly adjusted, for in 

 these objectives the focal length is generally very short, 

 and they approach, therefore, near enough to intercept 

 the outwardly refracted rays. 



With the exception of these differences of shading 

 (which are merely dependent upon the shape of the 

 corpuscle), the red corpuscles present a perfectly 

 homogeneous appearance, and exhibit in the fresh 

 condition no tendency to separate into the two parts 

 of which, as the study of the action of reagents will 

 show, they in reality consist. But there may gener- 

 ally be noticed, even in a preparation which lias been 

 made with the greatest care, a red corpuscle here and 

 there which varies from the prevailing form, having 

 become more globular, and at the same time rather 

 smaller in diameter; some of these retain a smooth 

 contour, whilst others, especially those near the edge 

 of the preparation, have a jagged or crenate margin, 

 as if set with little projections, and these may also be 

 seen on the surfaces of the corpuscles by carefully ad- 

 justing the microscope. This change of form, which 

 is very characteristic of the mammalian red cor- 

 puscles on exposure, seems to be generally caused by 

 a shrinking of the corpuscles, induced by an increase 

 in the density of the plasma in which they float; it 

 may always be produced by adding salt to blood. 



Turning now our attention to the white corpus- 

 cles, not more than two or three of which are to be 

 seen in each field of the microscope when the ordi- 

 nary high power is being used, and which, as before 



