which, involves the retina or to direct action of light on the iris. The latter view was the favorite one, but no efforts were made to characterize the elements of the iris on which the light acts, and so to clear up the physiological conditions of the phenomenon.
"Stick to the facts!" impetuously shriek many biologists when some luckless fellow insists that scientific method, with the principles of logic at its foundation, requires careful and incessant attention in biological investigation. In this case the sequel shows that there can be no doubt of the truth of any of the statements made during the whole fifty years. But vociferous discussion could not remove the contradictions, and experiment only multiplied them. Confusion is as consistent with facts as harmony is. It was the lack of a clear logical analysis of all the conditions of the problem that led to the contradictions. Nothing was demonstrated until these were removed, and it is an important fact that they were finally removed, not by disputing them, but by reproducing the conditions of the contradictory experiments and incorporating the contradictions themselves into the final solution of the problem.
Steinach, by his recent experiments, demonstrated that the sensitiveness of the iris varies immensely in different individuals of the same species; that the iris of frogs, kept for days in glass cases, does not respond at all to alternate shading and exposure to diffused daylight, but slightly to concentrated gaslight, and gives a regular reaction of appreciable amount only on exposure to concentrated sunlight; that when frogs are kept for a long time in the dark the iris responds promptly to diffused daylight; but if, after the light has produced contraction of the pupil, the frog, instead of being put back in the dark, is left exposed to the light, the pupil gradually dilates in spite of the light, and after some hours acquires a state of comparative insensibility, so that moderate changes in the light produce no changes at all in the iris; and that the difference in pupillary reaction between frogs kept in the dark and frogs exposed continuously to light is greater in the excised than in the normal eye, greater still when the iris is isolated from the rest of the eye, and that, while in frogs of medium excitability of iris the isolated eye still responds to light after shading, the iris, when separated from the rest of the eye, no longer responds even to the strongest light. One would think that at least some of these preliminary conditions of success would have thrust themselves upon the attention of the earlier investigators if they were not altogether lacking in the qualifications of true scientists. They were probably no more lacking in analytical
- Investigations on the Comparative Physiology of the Iris. H. Pflüger's Archiv für Physiologie, vol. lii.