02 Mechanical Philosophij. [Chap. I. 



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 Jigiire 

 which we discover by means of vision, and that 

 which we perceive by touch. By means of these 

 investigations and discoveries he enabled philoso- 

 phers to account for many phenomena in optics, 

 of which the most learned had before given Yery 

 erroneous accounts, or acknowledged themselves 

 unable to furnish any satisfactory solutions. About 

 the same time some valuable experiments and in- 

 structive works on the seat and principles of vision 

 were published by M. de la Hire, M. le Cat, 

 M. Bouguer, and several other French philoso- 

 phers. To these succeeded the inquiries of Har- 

 ris, Porterfield, Jurin, Smith, and still more re- 

 cently of Reid * and Wells f . In particular, the 

 very difficult question of apparent magnitude and 

 distaiice has been treated with great ability by 

 Berkeley and Harris ; the phenomena of single 

 and double vision have been solved by several of 

 the persons above mentioned ; and many remark- 

 able fallacies of vision explained by Mr. Melvillcj 

 M. Bouguer, and others. 



The principles and laws of Colours have also 

 been much better understood since the commence- 

 ment of the eighteenth century than- before. On 

 this subject it seems now to be generally agreed, 

 that the immortal Newton fell into essential mis- 



* Inquiry into the Human Mind on the Principles of Common 

 t An Essay upon Sinj^le Vision, &c. 8vo. 1 792. 



