Spatially Determined Reactions 193 



75. Class IV: Reaction to an Image 



By an image is meant the perception of simultaneously oc- 

 curring but differently located stimuli as having certain spa- 

 tial relations to each other. Through its means, or that of 

 the nervous processes underlying it, there arises the possi- 

 bility of adapting reaction not merely to the location of a 

 single stimulus, but to the relative location of several stimuli. 

 Responses may thus be adjusted not only to the direction of 

 an object but to its form. On the basis of such adjustments 

 a whole new field of possible discriminations is opened up. 



The commonest arrangement for the production of a visual 

 image is the double convex lens, which collects the rays of 

 light diverging in their reflection from an object and brings 

 them together again upon the sensitive retina. The lenses 

 found in many simple invertebrate eyes seem, however, very 

 ill adapted to the image-producing function. It is probable 

 that they serve rather to intensify the effect of the light rays 

 by bringing them together, than to give a clear-cut image 

 (293). In the eye of certain invertebrates, such as the Nauti- 

 lus, a gasteropod mollusk, while there is no lens, the opening 

 admitting the light rays is so small that an inverted image 

 might be formed through it, such as may be obtained through 

 a pinhole. It is unlikely, however, that this eye is really an 

 image-producing organ. Hesse includes under image-form- 

 ing eyes only the camera or convex-lens eye, the mosaic eye, 

 and the superposition eye. The last is a peculiar form of com- 

 pound eye where light can pass from one section to another, 

 and where the image is formed by the cooperation of various 

 refracting bodies (176). 



The simplest and vaguest conceivable visual image would 

 be that of a visual field whose different parts should differ 

 in brightness. An eye capable of furnishing indications 



