Iv.— The Optical Correction of Objectives Used for Microspectroscopy

SYNOPSIS The optical densities of small absorbing particles have been measured, initially with the microscope system well corrected optically and then after the introduction of spherical aberration by variation of the immersion fluid refractive index. It was found that several times the Rayleigh limit of spherical aberration could be tolerated without affecting the accuracy of the results, provided the condenser aperture was kept considerably smaller than the objective numerical aperture, this being a necessary condition from other considerations. There was, moreover, a surprisingly small deterioration of image quality. The effects on absorption measurements caused by the presence of coma and central obscuration in the microscope objective are also discussed.