COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 337 



and the monochromatic image thus formed may even be permanently 

 fixed by a proper treatment with alum solution (Kiihne, '79, p. 299) 

 or platinum chloride (Stern, : 05). 



By means of the ophthalmometer and other optical appliances, Beer 

 has made an extensive study of accommodation in the eyes of the lower 

 vertebrates and cephalopod.s. This faculty of accommodation may in a 

 general way be related to image formation, since it is probable that an 

 eye that is capable of accommodating differently to near and to distant 

 objects must be able to form fairly distinct images of those objects. The 

 converse, however, is not necessarily true, since there are eyes which 

 appear to have considerable image -forming power, but show no mechan- 

 ism for accommodation. The evidence that the compound eyes of 

 insects can accommodate seems insufficient, and there is no evidence 

 of this power in the eyes of any of the other invertebrates except the 

 cephalopods, unless perhaps the movements of the eyes of certain cope- 

 pods may be considered as such. 



All the methods which have been enumerated for determining the 

 exact nature of the images formed by eyes have certain obvious defects. 

 Especially is this true in the more lowly organized types of eyes, where 

 comparisons with the human eye cannot be so closely drawn. A study 

 of structure alone cannot give an exact basis for this determination, 

 since it is not always easy to interpret the optical properties merely 

 from the structure. This is well illustrated in the variety of opinions 

 that have existed as to "mosaic" vision in insects and other animals 

 with so-called compound eyes. Empirical tests with fresh eyes lose 

 much of their value on account of the changes that take place in the 

 tissues immediately their blood supply is cut off and they are removed 

 from the animal. The most serious of these changes are loss of tonus 

 and coagulation of the fluids. The image on the retina, due to the 

 change in the visual purple, is not definite enough to be of much use for 

 settling this question ; and the inferences to be drawn from the action 

 of the eye in accommodation can be of only the most general nature. 



It has not been the purpose of the work described in the following 

 pages to furnish a more exact method of determining the precise image- 

 forming powers of eyes. The aim has been, rather, to treat the forma- 

 tion of images ft"om the point of view of their relation to the animal as 

 a living organism, — to determine in what way the ability to form a 

 more or less perfect image affects the responses of the animal to light, 

 and what relation, if any, this result has to the normal habits of the 

 creature, and to its behavior under experimental conditions. This 

 investigation was suggested to me by Professor G. H. Parker, the sug- 

 gestion being an outcome of his study of the photo tropism of the mourn- 



VOL. XLII. 22 



