324 PHYSIOLOGY CHAP. 



The arrangement is shown in the diagram (Fig. 147) : G represents 

 the eye of the subject ; B that of the observer ; M the mirror con- 

 sisting of one, or better three highly transparent glass plates ; A 

 the source of light. Part of the rays emanating from the flame 

 are reflected by the glasses into the pupil of the eye C ; the light 

 reflected from the fundus of this eye passes through M, subjected 

 only to a slight lateral displacement, follows the direction of the 

 mirror image a, and reaches the eye of the observer B, on which 

 he sees the pupil of C as a red field. 



From the ophthalmologist's point of view it is important, however, 

 not merely to see the fundus of the eye in a general way, but clearly 

 to distinguish all the anatomical details of the retinal surface. 



Theoretically this should be possible without artificial aid 

 when both eyes are einmetropic and their accommodation entirely 

 relaxed. In this case the parallel rays would unite in a focus on 

 the retina, both of the observed and of the observing eye ; then the 



one would distinctly see the 

 fundus in the other, and vice 

 versa (Fig. 148). 



Theoretically this is also 

 possible when one eye is my- 

 opic and the other hyper- 

 metropic to a corresponding 



FIG. 148. To demonstrate how two einmetropic J n __-_ __ J ortnrV rYiT/w1afirm ia 



eyes (when sufficiently illuminated with accom- degree, and accommodation IS 



modation relaxed) can each view the fundus of re l axec l j n froth as the diver- 

 the other. ,, , 



gent rays from the hyper- 



metropic eye are made sufficiently convergent by the dioptric 

 mechanism of the myopic eye to come to a focus in the retina of 

 the latter, or the convergent rays from the myopic eye may fall 

 on the retina of the hypermetropic. 



Finally, a distinct view of the fundus of an eye is theoretically 

 possible in both emmetropic and ametropic eyes, when one or both 

 are appropriately accommodated. But it is clear that in all these 

 cases a distinct view of the fundus can rarely be obtained, because 

 it is difficult to ensure that the one eye shall maintain such a 

 state of complete repose or degree of accommodation as is required 

 to give a distinct view of the fundus. 



Helmholtz solved the practical problem of ophthalmoscopy 

 by interposing between the observed and the observing eye a 

 plano-concave lens, by which a magnified and erect virtual image 

 visible to the observer can be formed. The ophthalmoscopic 

 method, as first devised by Helmholtz, is shown in the diagram 

 (Fig. 149). A is the observed, B the observing eye ; / the flame, 

 which is partially reflected on to A by means of the system of 

 glass plates ab which acts as a mirror ; d the correction lens, held 

 nearer to or farther from B, according to the refractive power of 

 the two eyes. 



