A seasonal alteration in the sequence of events in the bleaching of frog visual purple was re-investigated by alteration of the length of the light period in the normal diurnal cycle to which the experimental animals were exposed. Frogs collected during winter and summer seasons were maintained on a light-dark sequence which corresponded both to the season of their collection and to the alternate season. The visual purple extracted from the retinas of the several experimental categories were exposed to light and the sequence of the bleaching reactions was followed. Comparisons were made upon the position of the isobestic point and the final products of bleaching.
When the conditions of measurement, temperature, pH, and light exposure were constant seasonal differences did not appear, nor did the alteration of the light-dark cycle result in any modification of the bleaching pattern. Changes in pH and light exposure cause changes in the bleaching pattern which are identical regardless of season or alteration of the light-dark cycle.
JAMES C. PESKIN and GERALD J. MILLER, "Bleaching of Visual Purple," J. Opt. Soc. Am. 46, 1048-1051 (1956)