

THE CANADIAN JOURNAL. 



NEW SE RIES. 



No. L.— MAKCH, 1864. 



A NEW OPHTHALMOSCOPE FOR PHOTOGRAPHING THE 

 POSTERIOR INTERNAL SURFACE OF THE LIVING 

 EYE ; WITH AN OUTLINE OF THE THEORY OF THE 

 ORDINARY OPHTHALMOSCOPE. 



BY A. M. ROSEBRUGH, M.D. 



Read before the Canadian Institute, January l&th, 1864. 



Before entering upon the construction and mode of- using this 

 Instrument I propose : 



(1.) Dwelling briefly upon the optics of the eye, glancing at the cause 

 of the blackness of the pupil under ordinary circumstances and 

 the invisibility of the parts behind it, and 

 (2.) Giving an outline of the optics involved in the ordinary Ophthal- 

 moscope. 

 In order to make the subject as plain as possible, I have at the 

 outset summed up the leading optical principles involved, and that 

 they may be more readily referred to, I have arranged them in the form 

 of Definitions — ( 1 ), Rays of light incident upon highly polished plate 

 glass with parallel surfaces, are partly reflected and partly refracted. 

 If the plate glass is thin, the rays that are not reflected may practi- 

 cally be considered to pass through the glass unrefracted. 



(2.) There is a point in every double convex lens called the optical 

 centre, rays of light passing through which are either unrefracted or 

 are refracted parallel to their original direction. 

 Vol. IX. F 



