Selection of the wavelengths A, B, C, C', or D when making ozone measurements is
accomplished by rotating Q1 and Q2 levers to positions specified in a Table of Settings of Q
provided with the instrument. Thick, flat quartz plates mounted immediately in front of the first and
last slits (S1 and S5) are fixed to the Q levers. Depending on the direction in which the quartz
plates are rotated, the light beam passing through them is refracted upwards or downwards,
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thereby providing for wavelength selection. Changes in the refractive index of the
spectrophotometer quartz prisms due to changes in the temperature of the instrument are allowed
for by making slight adjustments to the settings of Q1.