B'2 Mechanical Philosophy, 



The theory and laws of vision have received 

 very great elucidation during the last age. Bishop 

 Berkeley, in his Essay fozoard a Theorij of Vision, 

 published in 1709, solved many difficulties w^hich 

 had attended the subject, and threw much new 

 light upon it. He distinguished more accurately 

 than any who had gone before him, between the 

 immediate objects of sight, and those of the other 

 senses, which become early and insensibly associ- 

 ated with them. He first showed that distance, 

 of itself, cannot be determined immediately by 

 sight alone; but that we learn to judge of it by 

 certain sensations and perceptions which are con- 

 nected with it. He led the way, also, in pointing 

 out the difference between that extension and 

 .figure which we discover by means of vision, and 

 that which we perceive by tonch. By means of 

 these investigations and discoveries he enabled 

 philosophers to account for many phenomena in 

 optics, of which the most learned had before 

 given very erroneous accounts, or acknowledged 

 themselves unable to furnish any satisfactory so- 

 lutions. About the same time some valuable 

 experiments and instructive publications were 

 made on the seat and principles of vision, by M. 

 De la Hire, M. Le Cat, M. Boltguer, and 

 several other French philosophers. To these suc- 

 ceeded the inquiries of Harris, Porter field, 

 JuRiN, Smith, and still more recently of Reid^ 

 and Wells/ In particular, the very difficult ques- 

 tion of apparent magnitude and distance has been 

 treated with great ability by Berkeley and Har- 

 ris; the phenomena of single and double vision 

 have been solved by several of the persons above 

 mentioned; and many remarkable fallacies of 



g Inifulry into the Human Mind on the Principles of Common SensCf &C« 

 r An Essay upon Single f^ision, &c. 8vo. J 7 92. 



