Fig. 1. Diagram of the epi-microspectrophotometric apparatus. The monochromatic beam contains: Q_m, Ealing 150 W optically stabilized Xenon-arc illuminator; F_s, heat filter and stray light filter; G, computer-controlled monochromator with entrance slit A_n and exit slit A_x; L_m, quartz condenser; A_f, field aperture; W, computer-controlled counter-rotating neutral density wheels; S_m, computer-controlled Uniblitz shutter; M, mirror. The adapting beam contains: Q_a, 45 Wquartz-iodine illuminator; F_h, heat filter; F_d, neutral filter; F_c, cut-off filter; L_a, telephoto lens; A_a, field aperture; S_a, Uniblitz shutter. The two collimated beams are combined in quartz beam-splitter B_q and inserted into the Leitz Pol-Opak epi-illuminator equipped with 620-257 illumination slide designed for low stray-light. A, aperture diaphragm; L_o, microscope objective, either Leitz 8X/0.18P acromat/215 or Zeiss Luminar-25 mm/0.15; L_e, GF10X eye piece; E, eye of completely intact butterfly mounted on Leitz UT-4 goniometric stage. The Leitz UV-MPV photometer head contains A_p, adjustable/centrable photometer diaphragm; F_p, stimulus-blocking filter; P, cooled Hammamatsu R928 photomultiplier.