EEFRACTION IN THE EYE 489 



subject's retina enter the pupil of the observer 

 when the eyes are near. 



Ophthalmoscope. 1. The eye of the observer 

 cannot be placed in the axis of the illuminating 

 pencil without shutting off the illuminating rays. 

 This difficulty was obviated by the invention of 

 the ophthalmoscope. 



Place the electric lamp at the same height as 

 the artificial eye, and a little in front of and to 

 one side of it, so that the axis of the illuminating 

 pencil shall be at right angles with the visual 

 axis of the artificial eye. In front of the artificial 

 eye set a clear glass plate at an angle of 45 to 

 the axis of the illuminating pencil. A portion 

 of the rays which fall upon this plate will pass 

 through the transparent glass and be lost. An- 

 other portion will be regularly reflected, and will 

 be thrown into the artificial eye. A portion of 

 the light returning from the interior of the ob- 

 served eye will be reflected by the glass plate 

 and lost. Another portion will be transmitted 

 through the glass plate in the direction of the 

 visual axis of the observed eye, and may be 

 received by the eye of an observer placed in this 

 axis, as shown in the preceding construction 

 (Helmholtz). 



2. Examine the Loring ophthalmoscope. Its 

 essential parts are (1) the mirror of concave 



