longer (higher intensity) wavelength until the difference in the intensities of the two wavelengths
becomes zero (measured by Mp). The reading of the instrument is expressed as the position of W
where the difference of the intensities measured at Mp is zero. A second prism (P2) recombines the
dispersed radiation onto the exit slit (S5) where it is measured by Mp. The use of the double
monochromator minimizes stray light scattered internally within the instrument. Thus, the difference
of intensities of the wavelengths, and not the absolute intensities of the single wavelengths, is
measured by Dobson spectrophotometers.