656 



THE SENSES 



the power of accommodation is abolished, and the pupil is dilated. This will materially 

 facilitate the examination; but it is quite possible to observe all the details to be presently 

 described without the use of this drug. The room being now darkened, the observer seats 

 himself in front of the person whose eye he is about to examine, placing himself upon a 



FIG. 469. Diagram to Illustrate Action of Ophthalmoscope when a Biconvex Glass is Used. 

 The figure d on retina of a is under ordinary conditions focussed at / and inverted. If the lens 

 b be placed between eyes, the image h is seen by the eye c as an enlarged image. (After Mc- 

 Gregor Robertson.) 



somewhat higher level. A subdued but steady light is placed close to the left ear of the 

 patient in the examination of the right eye. Guiding the mirror in his right hand, and 

 looking through the central hole, the operator directs a beam of light into the eye of the 

 patient. A red glare, called in practice the reflex, due to the illumination of the retina, is 

 seen. The patient is then told to look at the little finger 

 of the observer's right hand as he holds the mirror; to 

 effect this the eye is rotated somewhat inward, and at the 

 same time the reflex changes from red to a lighter color, 

 owing to the reflection from the optic disc. The observer 

 now approximates the mirror, and with it his eye to the 

 eye of the patient, taking care to keep the light fixed upon 

 the pupil, so as not to lose the reflex. At a certain dis- 

 tance, which varies with the refractive power in different 

 eyes, but is usually an interval of about two or three 

 inches between the observed and the observing eye, the 

 vessels of the retina will become visible as lines running in 

 different directions. The smaller and brighter red arteries 

 can be distinguished from the larger and darker colored 

 veins. An examination of the fundus of the eye reveals 

 the optic disc and the entrance of the blood-vessels, the 

 macula lutea, and the fovea centralis. No blood-vessels 

 are seen in the fovea. This constitutes the direct method 

 of examination, figure 468; by it the various details of 

 the fundus are seen as they really exist, and it is this 

 method which should be adopted for ordinary use. 



If the observer is ametropic, i.e., is myopic or hyper- 

 metropic, he will be unable to employ the direct method 

 of examination until he has remedied his defective vision 

 by the use of proper glasses. 



In the indirect method, figure 469, the patient is 

 placed as before, and the operator holds the mirror in 

 his right hand at a distance of 30 to 40 cm. from the 

 patient's right eye. At the same time he rests his left 

 little finger lightly upon the patient's right temple, and 

 holding the lens between his thumb and forefinger, two 

 or three inches in front of the patient's eye, directs the 



light through the lens into the eye. The red reflex, and subsequently the white one, 

 having been gained, the operator slowly moves his mirror, and with it his eye, toward or 

 away from the face of the patient, until the outline of one of the retinal vessels becomes 

 visible, when very slight movements on the part of the operator will suffice to bring into 



FIG. 470. The Ophthalmo- 

 scope. The small upper mir- 

 ror is for direct, the larger for 

 indirect, illumination. 



