DROP OF BLOOD. 



seen edgeways, and others at all degrees of obliquity ; some are 

 scattered separately, but others are grouped together in piles, 

 with their edges presented to the eye, having the appearance of 

 rouleaux of coin lying on their sides on a table, the faces of the 

 coins being more or less inclined to the surface of the table. 



The flat disc- shape form of the corpuscles was not recognised by 

 the earlier observers, who took them to be red spherules. The 

 cause of this error was not any defect of their observation, but 

 arose from their having previously washed the blood with water, 

 being ignorant that the immediate effect of the contact of water 

 with human blood is to change the form of the flat corpuscles into 

 that of little globes. 



74. The magnitude of these corpuscles, since the recent im- 

 provements of the microscope, has been very exactly measured. 

 Their diameters are found to vary from the 3125th to the 3000th 

 of an ineh : this small variation being due to their different states 

 of development, as will be presently explained. 



75. The blood consists of a transparent, limpid, and colourless 

 fluid, in which the solid particles already mentioned float, and 

 the redness of which arises altogether from the colour of the 

 corpuscles here described. A person, who may observe for the first 

 time these corpuscles with the microscope, is generally surprised 

 and disappointed to find that they are not red, but rather of a 

 yellowish colour, having a very faint reddish tint. This circum- 

 stance, however, is an optical effect of a very general class, which 

 has been explained more than once in our Tracts. When any 

 coloured medium is submitted to the eye, the depth of its tint 

 will always be diminished with the thickness of the medium, 

 which may be reduced to such a degree of tenuity as to render 

 its peculiar colour altogether imperceptible. We mentioned 

 formerly, as an example of this, the case of coloured wine, such as 

 sherry, viewed through a tapering Champagne glass. At the 

 upper part, where the eye looks through a greater thickness of 

 the liquid, the peculiar gold colour is strongly pronounced ; but 

 in going downwards to the point of the cone, the colour becomes 

 paler and paler, and at the very point is scarcely perceptible. It 

 is the same with the red corpuscles of the blood. When they are 

 seen singly through their very minute thickness, they appear of 

 the faintest reddish yellow ; seen in rouleaux edgeways, they are 

 redder ; but it is only when amassed together, in a stratum of 

 blood of some thickness, that they impart to the liquid the red 

 colour so characteristic of the blood. 



76. The disc -shaped form which thus characterises human 

 blood, is common to all species of animals which suckle their 

 young, with the single exception, so far as is known at present, 



101 



