190 THE PEESIDENt's ADDRESS. 



obtaining which, I went, for some time, abroad ; and it was 

 printed, I presume, in my absence from England. At all events, 

 I never even heard of it until it fell into my hands on its second 

 issue in 1848. There were no diagrams in it, so it could only be 

 comprehended through its language and numerical calculations 

 of which only the data and results were given. I was 

 puzzled to understand it, so I wrote him a note informing 

 him of the existence of my paper, and calling his attention to a 

 want of correspondence between some of his decimals and 

 and fractions, which I said I had no doubt was accidental. But 

 what I really hoped for was to get incidently some more precise 

 information about his figures, as on a fixst perusal there did 

 not appear to me sufficient data for the making of his calcu- 

 lations. I received the following answer : — 



" Sir, — -I have read, as you wished me to do, your two papers 

 on musGce, voUtantes in the Medical Gazette for 1845, and I observe 

 that you have obtained some of the general results I had pre- 

 viously published in the Edinburgh Transactions. I consider 

 the subject as an optical one, capable of an accurate investigation ; 

 and by principles of diffraction we may determine the exact size 

 and locality of any body within the eye that projects a shadow 

 in divergent light. 



There are two errors in my numbers." Having corrected them, 

 he concludes : — 



' ' Permit me to call your attention to your explanation of the 

 magnifiying effect of a small hole. The only possible explanation 

 is that the apparent magnitude of the object is increased by its 

 proximity to the eye, and the hole does nothing more than allow 

 the object to be seen distinctly ; when its apparent magnitude is 

 increased by being brought nearer to the eye. Had the eye 

 possessed the power of seeing the object AB " (referring to an 

 engraving in my paper, where AB, as a straight line very near 

 the eye, is looked at through a pinhole, and is drawn perpen- 

 dicular to the common axis of the eye and hole, which cuts it 

 at A) "distinctly, the image of the point B would have been 

 exactly on the same point of the retina as with the hole. 



I am, Sir, yours very truly, 



D. Brewster." 

 St. Leonard's College, 



Jany. 29, 1848. 



