EYE AS A CAMERA OBSCURA. 



331 



^ 



" Let A be a flame, whose rays are caught at an angle on a glass-plate C, the 

 rays will be thrown along the line CD, into the eye D, which will see an image of 

 the flame along the line DB ; but the rays reflected from 

 the retina passing out in the same line DC, will again 

 meet the plate C ; they will be in part turned towards 

 A, but in part also will traverse the glass-plate C, and go 

 to form a picture at B of the image on the retina ; but an 

 eye G, placed behind the glass-plate and on the line CB, 

 will meet these reflected rays, and will consequently see j 

 the posterior chamber of the eye D illuminated."* L "" 



The experiment is thus made : — " In a dark room, 

 with a single flame at the side of the experimenters, 

 and on a level with their eyes, the person whose eye 

 is to be observed holds a piece of glass (a microscope 

 glass slip), so as to catch the image of the flame on 

 it ; he then, by inclining the glass, brings the image of 

 the flame opposite the pupil of the observer's eye ; the 

 latter will then see the pupil of the observed eye lumi- 



nous, of a reddish-yellow bright colour A 



person may also see one of his own pupils luminous : standing before a looking- 

 glass, and seeing the image of the flame in the reflector with his right eye, let him 

 bring this image opposite the pupil of the left eye in the looking-glass ; the left 

 eye will then perceive the right pupil in the mirror luminous."! 



The very simple arrangement which has been described, is all that is required 

 to develop the phenomena to which I wish to refer ; and the speculum differs from 

 it only by employing four glass-plates instead of one, to increase the illumination 

 of the observed eye, and adding a double concave glass, to increase the distinct- 

 ness of the image on the observer's eye. 



The reflector of Coccius, plane or better concave, is an ordinary glass mirror, 

 with the silvering on the back scraped off, so as to leave in the centre a small 

 transparent circular spot, or with a hole bored through the centre of the entire 

 mirror. Through this transparent spot or aperture the observer looks from the 

 unsilvered side, directing the light reflected from the opposite surface into the 

 observed eye, while the pupil of the latter and of his own eye are in a straight line. 



The mirror is round, with a diameter of about two and a half inches ; and a 

 focal distance of about six inches. The aperture in the centre has a diameter of 

 about two lines. The arrangement of the lamp is substantially the same as when 

 Helmholtz's speculum is used, and the mode of action, so far as illumination of 

 the observed eye is concerned, similar. 



With either of the arrangements described, the interior of any eye can be 



* Edinburgh Monthly Med. Journal, July 1852, p. 41. f Op. cit., p. 42. 



VOL. XXI. PART II. 4 U 



