THE SENSES 



801 



from a point of the retina of the observed eye are rendered 

 parallel by its dioptric media, and are again brought to a 

 focus on the observer's retina. 



If the observed eye is myopic, the rays of light coming 

 from a point of the retina leave the eye, even when it is 



. 301. FIGURE TO ILLUSTRATE THE PRINCIPLE OF THE OPHTHALMOSCOPE. 



Rays of light from a point P are reflected by a glass plate M (several plates together 

 in Helmholtz's original form) into the observed eye E'. Their focus would fall, as shown 

 in the figure, at P', a little behind the retina of E'. The portion of the retina AB is there- 

 fore illuminated by diffusion circles ; and the rays from a point of it F will, if E' is 

 emmetropic and unaccommodated, issue parallel from E' and be brought to a focus at 

 F on the retina of the (emmetropic and unaccommodated) observing eye E. 



FIG. 502. DIRECT METHOD OF USING THE OPHTHALMOSCOPE. 



Light falling on the perforated concave mirror M passes into the observed eye E' ; 

 and, both E' and the observing eye E being supposed ^emmetropic and unaccommodated, 

 an erect virtual image of the illuminated retina of E' is seen by E. 



unaccommodated, as a convergent pencil; and the emme- 

 tropic non-accommodated eye of the observer must have a 

 concave lens placed before it in order that the fundus may 

 be distinctly seen. 



When the observed eye is hypermetropic, the rays emerg- 



