110 THE BLOOD. 



The peculiar form of the blood-corpuscles gives them a 

 very characteristic appearance under the microscope. "When 

 examined with a magnifying power of from 300 to 500 

 diameters, those which present their flat surfaces have a 

 shaded centre, when the edges are in focus. Before we were 

 in possession of the perfect instruments now used in micro- 

 scopic investigation, this spot was supposed to be a nucleus 

 having a constitution different from the rest of the corpuscle. 

 Now this is understood to be an optical effect, the result of 

 the form of the corpuscle; their bi-concavity rendering it im- 

 possible for the centre and edges to be exactly in focus at 

 the same instant, so that when the edges are in focus the 

 centre is dark, and when the centre is bright the edges are 

 shaded. 



As the blood-corpuscles are examined by the microscope 

 by transmitted light, they are quite transparent, and of a 

 pale amber color. It is only when they are collected in 

 masses that they present the red tint characteristic of blood 

 as it appears to the naked eye. This yellow or amber tint is 

 characteristic. A pretty good idea of it may be obtained by 

 largely diluting blood in a test tube and holding it between 

 the eye and the light. 



In examining blood under the microscope, the corpuscles 

 are seen in many different positions ; some flat, some on their 

 edges, etc. This assists us in recognizing their peculiar 

 form. 



It has been observed by microscopists that the blood- 

 corpuscles have a remarkable tendency to arrange themselves 

 in rows like rouleaux of coin. This has attracted universal 

 attention, and for a long time was not satisfactorily explained. 

 Robin has lately given us what seems to be the true explana- 

 tion of this phenomenon. 1 This observer has shown that 



1 ROBIN, Sur quelques Points de VAnatomie et de la Physiologic des Globule* 

 Rouges du Sang. Journal de la Physiologic de THomme et des Animaux, 

 Paris, 1858, tome i., p. 295. 



