Notice of a peculiarity in Vision. 267 



next object is to interpose a glass of such a construction as 

 shall refract the rays in the vertical plain alone, that is, coun- 

 teract the too great vertical convexity of the lens of my eye. 

 To accomplish this, accurately, is no small difficulty. Let 3, 

 1, 4, 2, be the crystalline lens, of which the straight line 

 1 — 2 is a vertical section, and the curve 1 — 2 the vertical 

 curve, and let the straight line 3 — 4 be a horizontal section, 

 and the curve line 3 — 4 be the horizontal curve. Now the 

 points where the greatest refraction is needed are 1 and 2, 

 whence the refractive energy ought to decrease gradually un- 

 til it should actually disappear at the points 3 and 4, where 

 nature refracts correctly. (I, of course, speak here rather of 

 a diminution than an increase of refraction at the points 1 

 and 2.) To accomplish this, I conceive that I need a diffu- 

 sing glass composed of sections of a spheroid, whose trans- 

 verse axis is many times greater than its conjugate, but I 

 know not that such a glass could be ground very readily. I 

 have therefore been contented to procure piano cylindrically 

 concave glasses, whose single curve is about equal to No. 5 of 



l common spectacles. These 



spectacles I caused to be 

 made in Philadelphia a few 

 months since. 



The assistance which they 

 render my eyes is consider- 

 able, enabling me to read 

 (Greek more especially than 

 English on account of its 

 more numerous horizontal strokes,) at a distance of three or 

 four inches beyond the power of the naked eye. 



Their power on my eye, in viewing objects near at hand, 

 is a little too great, as they shorten the vertical dimensions 

 of objects a little beyond their true proportion. Rectangu- 

 lar objects near at hand, unless one of their linear dimen- 

 sions correspond with the greatest linear dimensions of my 

 own body, that is, ordinarily with a vertical plane, are liable 

 to be slightly disturbed. These difficulties would doubtless 

 be, in a great degree, remedied, could I procure the spheroi- 

 dal glasses mentioned above. 



Note. — If such glasses as are mentioned above, can pos- 

 sibly be ground, I should be happy to be informed where, and 

 by whom. C. E. G. 



Princeton Theol. Seminary, Feb. 29, 1828. 



