OBSERVING THE INTERIOR OF THE EYE. 689 



screen between the light and the person under observation, to prevent 

 any annoyance arising from the intensity of the light. The observer 

 must then place himself close in front of the person whose eye is un- 

 der observation, hold the glass in the manner described, and move it 

 about till the reflection of the light falls, upon the eye. The illu- 

 minated pupil will then be seen through the glass, and appear of a 

 reddish color. 



But, in order to see the separate parts of the retina distinctly, it 

 is necessary to make use of lenses adjusted to the sight of the ob- 

 server, and the refractive power of the eye under observation ; and 

 the result of such a combination is a perfect ophthalmoscope. The 

 glass, again, has been replaced with advantage by a mirror, generally 

 a concave mirror, with an aperture in the centre, through which the 

 observer looks. Fig. 4 shows the method of using this apparatus, 

 constructed after Ruete's plan. The light is placed near the person 

 under observation, A. The rays emitted fall upon the concave mir- 

 ror, d, which reflects them into the eye under observation. The ob- 



FlG. 4. 



server, _Z>, looks through the aperture in the concave mirror, and 

 moves the two lenses, m and Z, till they are in such a position that a 

 distinct image of the retina appears. 



We are now in a position, with the aid of the ophthalmoscope, to 

 make a thorough examination of the retina. Fig. 5 gives a toler- 

 able representation of all that we are able to distinguish of the image. 

 The background of the whole is of a dull red, while the point where 

 the optic nerve enters is distinguished as a round, bright spot, and we 

 may see rising out of its midst the retinal vessels, arteries, a, and 

 veins, , which extend over the entire retina. The yellow spot also, 

 the point of most distinct vision, may be distinguished as a small 



bright spot. 



VOL. ix. 4-t 



