SENSE ORGANS. I2 , 



thing — the object— outside in space. The object, how- 

 ever, is the facsimile of the image. It is as if the retinal 

 image were projected outward into space and appeared 

 there as an external image, which we interpret as an 

 object. I said as if. Really the retinal image is the 

 sign of the definiteness of the impression. For the 

 . molecular changes in the retina are graduated in degree 

 and kind exactly as the light is graduated. The light 

 image is a sign of an invisible molecular image. It is 

 this perfectly definite impression, this invisible molecu- 

 lar image, which, by the brain or by the mind, is referred 

 outward into space and interpreted as an object. 



This law is so fundamental that we stop a moment 

 to show that it is no new law specially enacted for the 

 sense of sight, but only an extreme modification of a 

 general law of sense-perception. 



Comparison with Other Senses. — We have al- 

 ready seen that stimulation of any sensory nerve in its 

 course is referred by the brain to the peripheral extrem- 

 ity. If the ulnar or the sciatic nerve is pinched any- 

 where in its course, pain is felt- not at the place, but in 

 the fingers or toes where the nerve is distributed. In 

 the case of an amputated leg, pinching the end of the 

 nerve in the stump causes pain in the toes where the 

 nerve is naturally distributed, even though there be no 

 longer any foot at all. In ordinary sensory nerves, 

 therefore, stimulation of any part of the nerve is referred 

 to the peripheral extremity. Now, the optic nerve differs 

 only in the fact that the impression is referred beyond the 

 peripheral extremity and out into space. 



This seems a great difference, but remember the grad- 

 ations already spoken of (page 96). In touch and taste 

 the reference is only to the peripheral extremity, because 

 the necessary condition of sensation is direct contact. In 

 smell we have indeed a sensation in the nose, but we 



