THE OPTHALMOSCOPE 



691 



ciliary muscle is thereby paralyzed, 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 with- 

 out the use of this drug. The room being now darkened, the observer seats 



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 focused 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 McGregor Robertson.) 



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



upon a 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 distance, 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 

 hypermetropic, 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 



FIG. 470. The Ophthal- 

 moscope. The small upper 

 mirror is for direct, the 

 larger for indirect, illumi- 

 nation. 



