20
Measuring Retardation from 1
λ
to 4
λ
Observing Extinction Position
○
Observe the position where the part of the specimen to be
measured becomes darkest by rotating the stage under crossed
polars.
Observing Subtraction Position
○
Rotate the stage 45
º
to the diagonal position where the specimen
appears to be at it’s brightest. Insert the quartz wedge into the slot
and optical path; verify the interference colour of the part to be
measured changes towards the lower order. If the colour changes
towards the higher order, rotate the stage another 90º.
Measuring Retardation
○
Slide the quartz wedge along the length of the slot, and the
interference colour will change.
○
Stop sliding the quartz wedge where the dark band covers the part
of the specimen to be measured. Remove the specimen and
compare the interference colour (at the same position without the
specimen) with the Interference Colour Chart to ascertain the
amount of retardation.
○
Measuring the retardation or ascertaining by interference colour
should be done with field of view diaphragm stopped down to the
part to be measured.
Note:
With monochromatic light the coloured fringes are seen as
alternating dark and bright bands.
