LIII. Normal ophthalmoscopy, indirect method. 



/. Appliances. The same as in exercise LII, with the addi- 

 tion of a lens of + 12 D. to -f 20 D. 



2. Operation. With the model or eye to be observed, the 

 light and the observer arranged as in exercise LII, 

 direct the light reflected by the mirror into the observed 

 eye and find the red reflection of the fundus of the eye. 

 Hold the lens between the thumb and index finger and 

 place it directly between the mirror and the eye under 

 examination, and at a distance from the latter of 6-8 cm. 

 Be careful that the center of the lens corresponds to the 

 center of the pupil and that the plane of the lens is per- 

 pendicular to the line of vision. 

 j. Observations. 



a. Observation of the emmetropic eye. 



(\) The rays of light emerging from the observed eye 

 are focused by the convex lens, which the observer 

 holds, and form an aerial image of the retina. If 

 a + 12 D. lens be used, and if its optical center be 

 held 8 cm. from the anterior surface of the cornea, 

 how far from the cornea will the aerial image be 

 formed ? 



(2) Trace in the image all of the structures enumer- 

 ated in the direct method. Is the image erect or 

 inverted ? Is the field larger or smaller than one 

 sees in the direct method? Are the structures mag- 

 nified or the reverse? Account for all phenomena, 

 representing the optics of the case with a figure. 



(3) Does a change in the distance between the cornea 

 of the model or eye and the lens which the observer 



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