relative to physical Optics. 13 
bodies with little or no resistance, as freely perhaps as the wind 
passes through a grove of trees. 
The observations on the effects of diffraction and interference, 
may perhaps sometimes be applied to a practical purpose, in 
making us cautious in our conclusions respecting the appear- 
ances of minute bodies viewed in a microscope. The shadow of 
a fibre, however opaque, placed in a pencil of light admitted 
through a small aperture, is always somewhat less dark in the 
middle of its breadth than in the parts on each side. A similar 
effect may also take place, in some degree, with respect to the 
image on the retina, and impress the sense with an idea of a 
transparency which has no real existence: and, if a small portion of 
light be really transmitted through the substance, this may again 
be destroyed by its interference with the diffracted light, and pro- 
duce an appearance of partial opacity, instead of uniform semitrans- 
parency. Thus, a central dark spot, and a light spot surrounded 
by a darker circle, may respectively be produced in the images of 
a semitransparent and an opaque corpuscle; and impress us with 
an idea of a complication of structure which does not exist. I11 
order to detect the fallacy, we may make two or three fibres 
cross each other, and view a number of globules contiguous to 
each other ; or we may obtain a still more effectual remedy by 
changing the magnifying power ; and then, if the appearance 
remain constant in kind and in degree, we may be assured that it 
truly represents the nature of the substance to be examined. It 
is natural to inquire whether or no the figures of the globules of 
blood, delineated by Mr. Hewson in the Phil. Trans. Vol. LXIII. 
for 1773, might not in some measure have been influenced by a 
deception of this kind : but, as far as I have hitherto been able to 
examine the globules, with a lens of one-fiftieth of an inch 
