SENSE OF SIGHT. 



579 



Circulation of Blood in the Retina. Not only is it possible to 

 see the shadow of the retinal blood-vessels, but the movement of 



FIG. 358. Diagram of the direct method with the formation of an upright image. 

 Rays from the source of light L are received upon the concave mirror M, and con- 

 verged upon the observed eye Obd., within which they cross and illuminate an area 

 of its fundus. From an area A B thus lighted, rays pass out of the pupil (parallel 

 if it be emmetropic, as here represented) through the sight-hole of the mirror, and, 

 entering the observer's eye, Obr., are focussed upon the retina. An image is there 

 formed as though the object seen was at a great distance, and the perceptive centers 

 project it into space as though the object was at some arbitrary distance (e. g., 25 

 cm.). By the laws of magnification by a simple lens the image is embraced between 

 the lines passing from the optical center of the magnifying-lens (the refracting 

 system of the observed eye), through the extremities of the object, and has the size 

 A' B', A" B", etc., according to the distance of projection (Randall). 



the corpuscles within these vessels can also be seen if the eye is 

 directed toward the sky. They appear as bright little bodies, 



Obr 



FIG. 359. Diagram of the indirect method, giving an inverted image : rays from 

 the source of light I/, converged toward the observed eye, Obd by the concave mirror 

 M, are intercepted by the lens Obj, and after coming to a focus diverge again and 

 light up the fuudus. From a part of the illuminated fundus A B rays pass out of 

 the pupil to be again intercepted by the lens 0, and form an inverted real image at 

 its anterior focus A' B'. This real image is viewed by the observer's eye behind the 

 sight-hole of the mirror with the aid of a magnifying-lens Oc, and is seen enlarged, 

 as at A" B" (Randall). 



moving rapidly and uniformly through the field. If cobalt glass 

 is held in front of the eyes, the corpuscles are more readily dis- 



