382. Dr. M.C. White, Diffraction in Microscopic Vision. 
line above a real opake line is often visible. This peculiarity of 
microscopic vision frequently makes a fine opake line appear 
double. 
If, to avoid seeing the spurious bright line above a real opake 
line, and also to diminish the breadth of the fringes formed by 
diffraction from the lower part of the line, we bring the lower, 
part of the object into focus we encounter another difficulty in 
resolving a series of lines. The light transmitted does not form 
a sufficient angle and the space between the lines, if they have 
much thickness, appears dark, as is the case with natural vision 
when looking into a long tunnel. 
_ I will now bring together some practical conclusions deduced 
from the above discussion and more or less comfirmed by ex- 
ment. 
First. If a minute object, whether a speck or a line, having 
an appreciable depth, is examined in the microscope, the effect 
of diffraction is to increase its apparent breadth. A speck ap- 
pears larger than it really is, a line or thread-form object appear 
wider than it should be and the edge of a diatom appears 
and indistinct. These facts are recognized by Hartung, “Das 
fil ” &c., § 247, but I am not aware that any writer has 
given an explanation of these facts. 
dly. A single line under favorable circumstances appears 
structures are examined. Cells having but a single wall may 
appear enclosed with double or triple walls, and opake molecules 
may appear as though enveloped by a cell wall. The pract 
eye will doubtless learn to detect such fallacies. ; 
When the first band of Nobert’s test, the lines of which are 
at the distance of ;,1,;; of an inch, is viewed in such a position 
that the lines appear intensely black, these black lines are ne 
the real grooves cut by the diamond but the spaces acini 
lines occupied by dark fringes produced by diffraction, wo" 
the bright are the bright lines of double rege wai 
the real lines, as explained above by means of fig. 4. By 
ing the objective nearer to the lines the black lines disappea™ 
and light spaces take their places, while the fine cuts of 
the dia- 
by sun- 
seg ee 
