THE BLOOD. 



35 



turned so that its edge meets the eye. This prominence cor- 

 responds to the ordinary nucleus of other elementary bodies or 

 cells. In this position the peculiar shape of the corpuscles, 

 with their constricted centres and rounded extremities, has 

 suggested a comparison between them and the little cakes 

 known as lady's-fingers. (See Fig. 13.) 



It is obvious also that this varying thickness of the disk will 

 have some effect upon the microscopic image, for the whole 

 superficies cannot be in focus at one time, even when the cor- 



FIG. 13. Red corpuscles of 

 the frog. (Rollett.) 



FIG. 14. Human red blood-corpuscles : a, globules 

 showing the double contour; 6, globules turned on 

 edge ; c, the same in rouleaux like coin. (Rollett.) 



puscle is turned flatwise to the eye. There will be some differ- 

 ence between the level of the thickest and thinnest portions. 

 As a result, when one is dark the other is bright, when one is 

 well defined the other is blurred. This statement serves for an 

 explanation of the double contour that is sometimes observed 

 in human blood (see Fig. 14), though it has also been offered 

 in support of the theory that the semi-solid and elastic matter 

 of which the disk is mainly composed has an external envelope 

 or limiting membrane of different density. It is to be remem- 

 bered, however, that the property of double refraction which 

 explains the double contour, belongs to all transparent bodies 

 that have rounded edges, such as drops of water or oil, in 

 which cases there is plainly no enveloping or peripheral wall. 

 When the lens and eye-piece are suitably combined, as in the 

 best microscopes, the double marking is often difficult or im- 

 possible to discover. On the other hand a poor optical com- 

 bination will generally exhibit it to an unpleasant degree, and 



