In ultraviolet microbeam irradiations, characteristics of the monochromator and lamp affect the spectral composition of the ultraviolet light and probably the biological results.

Biological conclusions recently published concerning ultraviolet (u.v.) microbeam irradiation of spindles are different from those we previously published. Several technical differences between the two sets of experiments were investigated. The spectral distributions in the light emitted from mercury-arc, xenon-mercury-arc, and xenon-arc lamps were measured, as were the spectral distributions after the light from these lamps passed through a monochromator that was set to various wavelengths and various half-band-widths. Both the source of the u.v. light and the half-band-width of the monochromator influence the spectral distribution of the light leaving the monochromator: depending on the conditions, the light leaving the monochromator is not necessarily of the same wavelength as that to which the monochromator is set. Differences in these aspects of the experiments could easily give rise to the different biological conclusions reached in the two sets of experiments.