Dr. M. C. White, Diffraction in Microscopic Vision. 381 
If z=0 then y=0, or if the object has no thickness there will 
be no fringe when the object is seen in focus. But if such an 
object having absolutely no thickness is a little out of focus it 
will be bordered by a fringe. If a=4 then y=4; i.e, = 
sstozx inch, y=syig, inch. If a=4i, y=2A. If c=94, y=3h. 
If z=164, y=4i. 
? 
. If xr=4Fi, q =°7i, If «<=+i, y="6i, If x=}i, y=4hi. If c= }4, 
y=h. z=,)4, y=}, &. From this reasoning it appears 
that if the real depth of a line is c= y4=y5s's07 inch the breadth 
of the line including the first fringe on either side cannot be less 
than ;53';55 of aninch. If the lines are ruled deeper the fringes 
will be broader, and it becomes an important question to know 
how we are to overcome this obstacle to the resolvability of series 
of lines at the smallest distances appreciable. If instead of em- 
Here theory beautifully coincides with observation, for in just 
those conditions lines are resolved in the microscope, which defy 
forined above the object, and the light would gradually fade away 
from the border towards the centre of the shadow. But by rea- 
ite sides into 
tays should coincide, or the object glass should bs 
be free from spherical aberration, but this result 
is only approximately secured in ordinary achr 
Matic objectives. The result ee the — of 
Vision has a certain depth, so that il avery thin © 
object is carefully tenes : 1 the microscope, this spurious bright 
