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 effeet 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 inch: 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 
