PICTUEES ON THE RETINA. 51 



in the state between waking and sleeping the 

 intensity of the impressions approaches to that 

 of visible objects. With persons of studious 

 habits, who are much occupied with the opera- 

 tions of their own minds, the mental pictures are 

 much more distinct than in ordinary persons ; 

 and in the midst of abstract thought, external 

 objects even cease to make any impression on 

 the retina. A philosopher absorbed in his con- 

 templations experiences a temporary privation of 

 the use of his senses. His children or his 

 servants will enter the room directly before his 

 eyes without being seen. They will speak to 

 him without being heard ; and they will even try 

 to rouse him from his reverie without being 

 felt ; although his eyes, his ears, and his nerves 

 actually receive the impressions of light, sound, 

 and touch. In such cases, however, the philoso- 

 pher is voluntarily pursuing a train of thought on 

 which his mind is deeply interested; but even 

 ordinary men, not much addicted to speculations 

 of any kind, often perceive in their mind's eye 

 the pictures of deceased or absent friends, or 

 even ludicrous creations of fancy, which have 

 no connexion whatever with the train of their 

 thoughts. Like spectral apparitions they are 

 entirely involuntary, and though they may have 

 sprung from a regular series of associations, yet 

 it is frequently impossible to discover a single 

 link in the chain. 



If it be true, then, that the pictures of the 

 mind and spectral illusions are equally impres- 

 sions upon the retina, the latter will differ in no 

 respect from the former, but in the degree of 

 vividness with which they are seen; and those 

 E 2 



