Mr. W. G. Horner on the Vascular Spectrum. 265 



produce the image occupy a surface parallel to that which is 

 the seat of vision. 



11. Every indication concurring to prove that a condensed 

 light thrown on the sclerotica was able to traverse its sub- 

 stance, and that of the vitreous humour, with a concentration 

 sufficient to cast the shadows of these fine vessels upon the 

 retina, their distance from it, and their size, were thus deter- 

 mined. A scale of inches and parts being held along a wall, 

 distant 15 feet from my eye, I observed that the projection of 

 one of the principal vessels in fig. 2. occupied a breadth of 

 about half an inch ; and that they moved over the space of 

 one foot, while the focus of a lens traversed the whole extent 

 from one angle of my eye to the other. Admitting this to be 

 150°, and the distance of the retina from the optical centre of 

 the eye f * of an inch, we have 15 feet : f inch : : \ foot 



: -% Q inch, whence ^ cot — — = ^L inch, very nearly, is the 



distance of these vessels from the retina ; and \ foot : \ inch 

 : : jy inch : T ^ n inch, very nearly, is the diameter of the me- 

 dium vessels. 



12. The agreement of this estimate of distance with the ob- 

 servation of Art. 9. is very satisfactory ; for the whole ex- 

 posable angular extent of the eye being about 240°, we have J-^ 



tan — = ^- = ^j inch very nearly, for the entire range 



of the shadow of each vessel. Now that this exceeds the 

 greatest breadth of the punctum caecum in my own eye, I am 

 assured by direct experiment. For I find that an oval, 12 inches 

 in height by \0\ in breadth, and whose vertical major axis is 

 at the distance of 2 feet from a mark in a horizontal line cut- 

 ting the oval at one third of its axis from the vertex, just dis- 

 appears when the mark is viewed from a distance of 8 feet 

 perpendicularly to the plane of the oval. Hence by a qua- 

 dratic equation, viz. (•22 + ig) (*69 — 4?/) = y 2 or ( - 10+4?/) 

 ("81— 4j/) = y 2 , according as Young's or Herschel's estimate 

 of the optical centre is adopted, we find y = '16 or '19 

 inch = the distance of the centre of the optic nerve from 

 the axis of vision. The breadth of the punctum caecum will 

 be at most (2 feet: lOf inches : : *19:) -085 inch. D. Bei- 

 noulli's estimate is stated at \ the diameter of the eye, which, 

 by Dr. Young's measurements, is - 13 inch. Priestley's ac- 

 count of Bernoulli's data is imperfect; so that the cause of 

 this discrepancy is unknown to me, the Petersburg!! Trans- 



• Sir J. Ilrjrscbel places the optical centre in the centre of the iris; 



Dr. Young at -22 inch from the front of the cornea. I take the ineilitiiu 

 a» sufficient for the ohject in view. 



Third Series. Vol. 4. No. 22. April 1 834. 2 M 



