LOGICAL METHOD IN BIOLOGY. 375 



power than any other set of investigators, but they had certainly 

 not made the most of it. Steinach exhausted the conditions by 

 taking into account, one at a time, the different degrees of inher- 

 ent excitability, effect of exposure to and exclusion from light, 

 and the reactions of the normal eye, the excised eye, and the iso- 

 lated iris, against the different degrees of light, thus : 



{ Diffused light. 

 'Normal eye. < Concentrated gaslight. 

 Excluded from light. \ j,^^;^^^ ^^^ ^ Concentrated sunlight. 



Isolated iris. 



Frog's eye. ^ 



High excitability. 



Exposed continuously 

 to light. 

 Medium excitability. 

 Low excitability. Etc. 



These do not include all the conditions which he detected, but 

 they are sufficient to indicate the difference between his method 

 and that of his predecessors. The modifying conditions were not 

 discovered in the order in which they appear in the table, but 

 tabulation shows very quickly whether or not they have been 

 exhausted. 



When all the favorable conditions were combined there in- 

 variably resulted a characteristic contraction of the pupil, on 

 exposure to light, whether the object experimented on was the 

 normal eye, the excised eye, the isolated iris, or the isolated iris 

 deprived of its ciliary rim. In other words, the contraction of the 

 pupil in the excised eye of fishes and amphibia does not depend 

 on an intraocular reflex involving the retina, but on the direct 

 influence of light on one or more of the elements of the inner or 

 pupillary part of the iris. It had been suggested that the phe- 

 nomenon was due to the action of light on the endings of the 

 nerve fibers in the sphincter muscle of the iris. Steinach removed 

 this suggestion from the group of remaining possibilities by para- 

 lyzing the nerves of one eye of an animal with atropine and leaving 

 the other normal, and showing by comparative tests that the two 

 eyes continue to act alike. He showed by a special experiment 

 that the posterior pigment layer of the iris has nothing to do with 

 its contraction. The branched or stellate pigment-cells the chro- 

 matophores in the front part of the iris were possible factors in 

 the problem. They were known to undergo changes due to the 

 action of light. Light causes a redistribution of the pigment 

 within the cell, causing it to collect at the center. When the eye 

 of an animal which has been kept in the dark is alternately 

 shaded and exposed to the light, there follow a prompt alternate 

 dilatation and contraction of the pupil. This process can be car- 

 ried on for some time before there is anj visible change in the 

 chromatophores ; at the end of half an hour or more the chro- 



