376 THE POPULAR SCIENCE MONTHLY, 



matophores are " contracted/' but the pupil contracts and dilates 

 as before. Therefore the contraction of the iris is independent of 

 the changes in the chromatophores. 



At this point physiological experiment had to be abandoned, 

 and it would have been extremely comfortable for Steinach to 

 do as one of his predecessors had done ride the rest of the way 

 on a cantering hypothesis ; but he appealed to histology. In his 

 effort to determine in what other parts of the iris there was 

 pigment, through which the light must produce its effect, he 

 found that his judgment was confused by particles of pigment 

 from the posterior layer, which were scattered at random over 

 his histological preparations. This difficulty was obviated by 

 removing the posterior layer of pigment before making the sec- 

 tions. After taking this precaution he showed that there is no 

 ordinary pigment in the stroma of the iris ; neither are there any 

 ordinary smooth muscle fibers like those in the iris of the higher 

 vertebrates. He found the sphincter muscle of the iris composed 

 of spindle-shaped pigmented cells. That these are really muscle 

 fibers he proved by their form, size, characteristic fibrillar struc- 

 ture, and function. It was impossible to observe directly the con- 

 traction of these fibers ; he adopted the indirect method of killing 

 the iris in the relaxed and in the contracted states and observing 

 the condition of the fibers in each. In the former they were 

 slender and narrow, in the latter shorter and thicker. The ciliary 

 muscle fibers are not pigmented, and this accounts for their being 

 indifferent to the light. His general conclusion is that light pro- 

 duces contraction of the isolated fish and amphibian iris by act- 

 ing directly on the fibers of the sphincter muscle through their 

 pigment. 



The striking characteristic of this investigation is the exhaust- 

 ive consideration and removal of alternative beliefs. His final 

 conclusion is only an inference, and derives its " certainty " from 

 the fact that it is the only belief that is left. In its relation to 

 this conclusion the evidence is circumstantial. If now the reac- 

 tion of the pigment and fibers could be directly observed, Stei- 

 nach's conclusion would be set down as a verified prediction. 

 Though unverified, it is unhesitatingly accepted, like so much of 

 our " knowledge," as an important truth ; for most minds its veri- 

 fication would add little or nothing to its certainty, and would 

 even deprive it of some of its interest. This inferential knowl- 

 edge forms a large part of scientific truth, and other instances 

 of it will appear in the following example of method in mor- 

 phology. 



Various ciliated organs of unknown function in different 

 mollusks had never been brought under the yoke of homology. 

 One of the most decisive tests in morphology for the determination 



