THE OPHTHALMOSCOPE. 



765 



of the patient, A that of the observer, and let a flame be placed at x. The rays of light pro- 

 ceeding from x impinge upon the obliquely placed plate of glass (S,S), and are reflected in the 

 direction of the dotted lines into the eye (B). The fundus of the eye appears in this position 

 to be brightly illuminated in diffusion circles around b. As the observer (A) can see through 



Fig. 549. 



the obliquely placed glass plate (S, S), and in the same direction as the reflected rays (x, y), he 

 sees the retina around b brightly illuminated. 



In order that this method be made available for practical purposes, we must, of course, be 

 able to distinguish the details, such as the blood-vessels of the fundus of the eye, the macula 



Fig. 550. 



lutea, the entrance of the optic nerve, abnormalities of the retina, and the choroidal pigment, 

 &c. The following considerations show us how to proceed in order to accomplish this. As 

 already mentioned, and as fig. 531 shows, a small inverted image is formed on the retina (c, d) 

 when we look at an object (A, B) ; conversely, according to the same dioptric law, an enlarged 

 inverted real image of a small distinct area of the retina (c, d depending on the distance for 

 which the eye was accommodated) must be formed outside the eye (A, B). 



Fig. 551. 



If the fundus of this eye be sufficiently illuminated, this aerial image will be correspondingly 

 bright. 



In order to see the individual parts of the retinal picture more distinctly, the observer must 

 accommodate his own eye for the position of this image. In such circumstances the eye of the 

 observer would be too near the observed eye. His eye when so accommodated is removed from 



