An optical wedge, consisting of two quartz flats coated with chromel, is mounted in the
instrument in front of slit S3. The position of the wedge is controlled by turning a graduated dial
located on top of the instrument. With the dial set at 0° the thin portion of the optical wedge is
positioned in front of slit S3 so that light passes through the optical wedge and slit S3 with
practically no loss of intensity. With the dial set at 300°, however, the S3 light beam is almost
completely absorbed by the thick portion of the optical wedge. It follows that there exists a
"balance" setting of the dial somewhere between 0° and 300° where the intensity of the light beam
passing through the optical wedge and slit S3 will have been reduced to the level of the intensity of
the S2 wavelength beam (or S4 wavelength beam if observations on C' wavelengths are made).
Now, for any given position of the dial the intensity of the light passing through the optical wedge is
reduced in a definite ratio which is determined during the original calibration of the
spectrophotometer. In order to measure the relative intensity of the two wavelengths on which
observations are made, then, it is necessary only to be able to detect the balance position of the
dial.