DIOPTRICS OF THE EYE. 



305 



lary accompaniment of the emotional states may occur even when 

 it is a matter of memory rather than immediate experience. The 

 explanation of this mydriasis can hard]y be obtained by experi- 

 ment, but reasoning from analogy we know that strong emotional 

 states are usually accompanied by more or less distinct inhibitory 

 effects on motor centers, and perhaps in this case the reaction is 

 most satisfactorily explained by attributing it to an inhibition of 

 the constrictor center in the midbrain. 



The Ophthalmoscope. The light that falls into the eye is 

 partly absorbed by the black pig- 

 ment of the choroid coat and is 

 partly reflected back to the ex- 

 terior. This latter portion is re- 

 flected back in the direction in 

 which it entered. Merely holding 

 a light near the eye does not, there- 

 fore, enable us to see the interior 

 more clearly, since in order to catch 

 the returning rays in our own eye it 

 would be necessary to interpose the 

 head between the source of light 

 and the observed eye. If, however, 

 we could arrange the light to enter 

 the observed eye as though it pro- 

 ceeded from our own eye, then the 

 returning rays would be perceived, 

 and with sufficient illumination the 

 bottom or fundus of the observed 

 eye might be seen. Arguing in this 

 way, Helmholtz constructed his first 

 form of the ophthalmoscope in 1851. 

 The value of the ophthalmoscope is 

 twofold: It enables the observer to 

 examine the interior of the eye and 

 thus recognize diseased conditions of 

 the retina ; it is also useful in detect- 

 ing abnormalities in the refractive 



surfaces of the eye. The principle of the instrument is well repre- 

 sented in the original form devised by Helmholtz, as shown schemat- 

 ically in Fig. 131, A . I represents the observed eye and 77 the eye of 

 the observer. Between the two eyes is placed a piece of glass inclined 

 at an angle. Light from the candle falling upon this glass is in part 

 reflected from the surface to enter eye I, and these rays on emerging 

 from the eye along the same line pass through the glass in part and 

 enter eve 77. In place of the plane unsilvered glass it is now cus- 

 "20 



Loring's ophthalmoscope. 



