166 On the Advantage of Periscopic Spectacles. 



ous observations of Mr. Jones, or to remark upon the 

 experiment by which he has deceived himself, because 

 all doubt of the advantage of the periscopic glasses may be 

 removed by the following direct comparative trial, which 

 any person v>ho chooses can repeat without difficulty. 



I have before me two glasses, each of 4 inches " posi- 

 tive focus," as proposed by Mr. Joiaes, the one double 

 convex, which in his judgment is pronounced to be " iii- 

 duiUabhj the lest and most convenienl that can he devised" 

 the other a concavo-convex, or meniscus, which bethinks 

 '' evidently the worst of the two for a spectacle-glass." 



When I fix the former at the distance for most distinct 

 vision opposite to a printed octavo page, and approach 

 my eve to the glass, I cannot without pain read quite 24 

 lines; but upon substituting the periscopic glass, tixed in 

 the same position, I can discern every word in the page, 

 which contain? 40 lines. 



The enlargement of the field of view observable in this 

 trial, is sufficient to evince the superior utility of the pe- 

 riscopic glasses ; but were there occasion to compare more 

 nearly the circular surfaces that may be seen with equal 

 distinctness by each, they would be found to differ by a 

 ratio as great as that of three to one. 



The difference is of course more evident in glasses of so 

 high power, than in those used by most long-sighted per- 

 sons for common purposes ; but it cannot be doubted 

 that in the latter ab«o a corresponding, though smaller, 

 inequality subsists, wherever there is the same dissimi- 

 larity of construction^ even when the focal distance is 

 longest. 



The advantage in question is, therefore, indisputably 

 proved by direct experiment; to the novelty also Mr. Jones 

 himself has unintentionally contributed very satisfactory 

 evidence ; but as to its importance, those only who have 

 the misfortune to labour vmder any defect of vision must 

 ultimately decide. 



I remain. Sir, 

 Your obliged humble servant, 

 March 30, 1S04. W. }I. WoLLASrOK, 



XXTX. Pro<;css. 



