630 POPULAR SCIENCE MONTHLY. 



where they may bring' with them some special addition to our stock of arts 

 or articles of culture. (Works, ii, p. 576.) 



Neither Madison nor any of the others had any conception 

 of modern immigration, and apparently never realized that their 

 moderate and, as they supposed, well-regulated encouragement 

 would bring it ahout. 



ILLUSIONS AND HALLUCINATIONS. 



By Prof. WILLIAM EOMAINE NEWBOLD. 



I HAVE already had occasion more than once to speak of the 

 development of a mental state from the stage which we term 

 idea to that which we term sensation. Before taking up the mat- 

 ter in hand it will be necessary to go into this question at some- 

 what greater length. 



We seldom have difficulty in discriminating an idea from a 

 sensation, but it is not easy to define the difference between them. 

 This is partly due to the fact that the differences are very com- 

 plex, and partly to the fact that they vary in the respective fields 

 of sensation, so that one can scarcely frame a definition for ideas 

 and sensations of vision that will also prove applicable to those of 

 sound, touch, and so on. Ideas of sound differ from the corre- 

 sponding sensations chiefly in intensity, but in the case of vision 

 a much more important distinction is drawn from the relation 

 sustained by visual ideas to what the eye actually sees. At the 

 present moment I am thinking of something I saw yesterday, but 

 what I see with my eyes is not in the least affected by that. 

 The two groups remain distinct, and it would seem as if an almost 

 impassable gulf parted them, so seldom does a bit of one become 

 confused with the other. This is not true of ideas of sound. If 

 they only become intense enough they may seem to blend with 

 real sounds — indeed, I often mistake an air of which I am think- 

 ing for the same air faintly heard. 



The distinction between sense-impression and idea really rests 

 upon intrinsic differences of this kind, but as they are so complex 

 I shall make use of a physiological distinction which for all prac- 

 tical purposes coincides with it. Sense-impressions are those 

 mental states which are primarily initiated by a current from the 

 outlying regions or periphery of the body, especially from the 

 organs of sense. Since these currents are usually due to the action 

 of physical forces upon the body, sense-impressions generally give 

 us information as to the condition of the material world. All 

 other mental states should be classed as ideas, even though they 

 simulate sensations so closely as to be scarcely distinguishable 

 from them. 



