110 Transactions of the Royal Microscopical Society. 
appears light. Proceeding toward the periphery of the corpuscle, 
a slight shade is seen to arise from the light centre, which, after 
somewhat increasing in depth, is gradually lost, to be followed by 
the high light, representing the convexity of the margin (Fig. 
59, a). This variation of light and shade is of course caused by 
the form of the corpuscle. The convex margin appears most 
illuminated at the highest part of the convexity, where the rays 
of light passing through it undergo no, or very little, refraction, 
while more or less shade must appear, by virtue of the refraction of 
light, at that part of the surface which inclines toward the centre, 
and forming a part of the concavity ; the centre finally being the 
thinnest portion of the corpuscle and very nearly flat, must, from 
the absence of almost any refraction, appear light. Owing to the 
rounded margin of the blood-corpuscle, its very outlines do not 
appear distinctly and sharply defined ; on the contrary, a delicate 
shade is observed at the very edge, which soon disappears in the 
high light of the convexity. No trace of the existence of a mem- 
brane or a membraneous layer can be discovered in the fresh blood- 
corpuscle of Man. When brought into the exact focus, it appears 
encircled by a distinct narrow ring of a pinkish tint, much lighter 
than the rest of the surrounding liquor sanguinis ; this phenomenon 
is probably owing to the corpuscle refracting the light toward its 
less refractive liquid medium. The appearance of the coloured 
blood-corpuscle of Man in exact profile is peculiar, and corresponds 
not to the bi-concave form, in which it is so often erroneously 
represented. Eecollecting that its body represents a minute round 
plate or disk, the peripheral portion of which, besides being convex 
and rounded at its border, is twice as thick as the central, which 
is concave, and further, that it is perfectly symmetrical at all points, 
it becomes evident that the outlines of its profile must be repre- 
sented by two straight and lines, connected at their ex- 
tremities by two semicircular ones ; and accordingly the side view 
of the corpuscle could not reveal the concavity of the central 
portion. But if a vertical section were made through the centre 
of the corpuscle, the outlines of the cut surface would be repre- 
sented by two slightly curved lines, which directing their convexity 
toward each other, are connected by semicircular lines, showing 
not only the bi-concave form of the central, but also the convex 
and rounded form of the peripheral portion of the body. 
The contour of the coloured blood- corpuscle in profile, though 
in reality corresponding to the above description, may nevertheless 
appear differently when the object is carelessly examined, and 
instead of being represented by two straight and parallel lines, 
connected by semicircular ones, its sides may appear bi-concave, 
resembling the outlines of a section through the centre, as before 
described. The cause of this optical illusion must be sought in the 
