Page:Popular Science Monthly Volume 9.djvu/717

Rh 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 under observation, hold the glass in the manner described, and move it about till the reflection of the light falls, upon the eye. The illuminated 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 observer, 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 mirror, d, which reflects them into the eye under observation. The observer



B, looks through the aperture in the concave mirror, and moves the two lenses, m and l, 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 tolerable 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, b, which extend over the entire retina. The yellow spot also, the point of most distinct vision, may be distinguished as a small bright spot.