In 1947, Francis Hughes Smith filed for a differential interference contrast patent
using paired Wollaston prisms,
with one prism located where phase plates are inside phase contrast objectives.
However, precisely manufacturing and aligning Wollaston prisms inside objectives is prohibitively tedious and expensive.
Nomarski's 1952 patent enabled locating DIC prismsoutside objectives,
where they can relatively easily be aligned.
DIC prism manufacture remains fairly fussy, and Nomarski's practice of licensing
only one microscope manufacture per country helped limit supply and keep prices high.
Wollaston
Nomarski
The front interference plane of an objective-side Nomarski prism
must coincide with the objective's back focal plane.
The upper wedge must be thinner than the lower wedge in a Nomarski prism,
but most drawings just have a diagonal straight line between corners.
Small but important difference.
What you need to look out for is the polarizer and analyzer.
The best option has a rotatable polarizer
with a separate quarter wave plate slider,
mostly meant for epi DIC but good without too.
The objective's prism and the condenser's prism must match.
I did try to use Nikon objective's prisms (made for the N2 condenser's prism) with M condenser's prism and they did not work.
Condenser prism are made to be mounted at precise distance from the condenser's lens, close to focal point.
Prisms have to be oriented 45 degrees from polarizer direction
(Polarizer is typically oriented left-right and analyzer back-front)
First, find objective prism's orientation:
remove both prisms from the light train,
and cross the polarizer and analyzer
(light through eyepieces is darkest),
now put the objective's prism in the light train,
remove an eyepieces and check for the black line:
it should be oriented at 45 degrees,
note: not all prisms show the black stripe when put between crossed polarizers
(the Nikon N1, N2 family of DIC prisms do not show that stripe, but mostly do).
Second, orient the condenser prism's black stripe in the same direction
(/ or \\) as the objective's stripe,
by removing the objective prism from the light train
inserting the condenser prism, and
turning that prism until its black stripe aligns with the objective's.
"let salt soak in water and let the water evaporate.
I just put my slide on the central heating to speed up.
But i was not happy with the forms, so i did it a second time.
Then mixed with alcohol."
PlasDIC is a special design of Köhler-DIC configuration
in which a slit in the direction of shear is placed symmetrically with respect to the optical axis in the FFP of the condenser.
Köhler-DIC uses an condenser aperture of the same size as objective aperture.
