374 THE POPULAR SCIENCE MONTHLY. 



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 multi- 

 plied 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 demon- 

 strated until these were removed, and it is an important fact that 

 they were finally removed, not by disputing them, but by repro- 

 ducing the conditions of the contradictory experiments and incor- 

 porating 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 con- 

 centrated 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, in- 

 stead 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 mod- 

 erate 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 investi- 

 gators 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. II. Pfliiger's Archiv fiir 

 Physiologic, vol. lii. 



