24
7
Accessories
7.1 Pinhole Stop
Replace one of the eyepieces with an eyepiece tube insert with a
pinhole stop.
Remove the Bertrand lens out of the optical path and the conoscopic
image can observed overlapping the orthoscopic image through the
binocular observation.
7.2
Interference Colour Chart
Examine the quartz wedge between crossed polarizers under white
light. It produces a range of coloured bands that form
Newton’s
colour scale,
which is reproduced commercially as the Michel Levy
Chart.
At the thin edge of the wedge the thickness and retardation are 0, all
of the wavelengths of light are cancelled at the analyzer resulting in a
black colour.
With increasing thickness, corresponding to increasing retardation, the
interference colour changes from black to gray to white to yellow to
red and then a repeating sequence of colours from blue to green to
yellow to red. The colours get paler, or more washed out with each
recurrence.
In the above image, the repeating sequence of colours changes from
red to blue at retardations of 550, 1100, and 1650 nm. These
boundaries separate the colour sequence into first, second and third
order colours.
Above fourth order, retardation > 2200 nm, the colours are washed
out and become white.
The interference colour produced is dependent on the wavelengths of
light, which pass through the analyzer, and the wavelengths that are
cancelled.
Eyepiece tube insert with
pinhole stop.
