in connexion with Vision. 243 



the image formed by it upon the retina. But it is obvious 

 that from such a mode of proceeding no conclusion could ever 

 be drawn. There ought to have been some standard, or some 

 fixed point introduced, by which to calculate the comparative 

 direction of the object ; or to enable us to estimate the truth 

 of the demonstration, by contrasting one line of visible direc- 

 tion with another. 



2. But it is in his experiment that the most remarkable 

 error is found: indeed, the conclusion which he has drawn 

 from it must have somewhat startled the reader. He affirms 

 that if the rays RC RL, proceeding from a distant point, be 

 cut off by a card (which may be represented by AB), and the 



n 



ray RL', alone enters within the eye, the image F will be seen 

 along the line FCR. But how can this, by any possibility, be 

 the case ? Is it not obvious that the card AB totally obstructs 

 vision in that direction ? if we * see along ' the line, it can only 

 be by seeing through the card ! An explanation seems to be 

 required how Dr. Brewster could have been brought to make 

 so extraordinary a statement. The error can only have been 

 occasioned by his proceeding on a wrong path in making this 

 inquiry. Unexpected conclusions are often forced upon us in 

 the study of mathematics ; and even when a strict course of in- 

 ductive reasoning is pursued in physical subjects, the results 

 are often strange, and apparently paradoxical: so that we need 

 not be surprised that a learned philosopher should occasionally 

 yield his assent to a proposition which an uneducated person, 

 guided solely by his common sense, would at once reject. But 

 it does not appear that the conclusion which Dr. Brewster has 

 drawn, in the present instance, is to be justified on the grounds 

 of its being founded either on mathematical demonstration, or 

 on correct induction. In explaining the result of his experi- 

 ment, he has mixed up with the facts a bare and unwarranted 

 assumption regarding the power which belongs to the retina. 



