VISION 203 



(c) Observation of the Retina in a Myopic Eye. Adjust the model 

 for myopia e. g., three dioptrics. 



(10) Are the retinal bloodvessels distinct? 



(11) What direction do the rays from the retina take on emerging 

 from the myopic eye: divergent, convergent, or parallel? 



(12) In which of these three cases would the normal eye be able 

 to get a clear image of the retinal structures? 



(13) In which case would a correcting lens be necessary? Should 

 one use a convex or a concave lens, and why? 1 



XII. NORMAL OPHTHALMOSCOPY (INDIRECT METHOD). 



1. Appliances. The same as in preceding exercise, with addition 

 of a lens of + 12 D. to +20 D. 



2. Operation. With the model of eye to be observed, the light 

 and the observer arranged as above, direct the light reflected by 

 the mirror into the observed eye and find the red reflection of the 

 fiindus of the eye. Hold the lens between the thumb and index finger 

 and place it directly between the mirror and the eye under examina- 

 tion, and at a distance from the latter of 6 cm. to 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 perpendicular to the line of vision. 



3. Observations, (a) Observations of the Emmetropic Eye. (1) 

 The rays of light emerging from the observed eye are focused by the 

 convex lens, which the observer holds, and forms 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 enumerated 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 

 magnified 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 holds alter the size of the 

 image? Account for observation. 



(b) Observation of the Hyperopic Eye. Adjust the model for 3 D. 

 of hyperopia. 



(4) Does an increase of the distance of the lens from the cornea 

 cause the image of the papilla to be altered in size? Account for all 

 phenomena. 



1 In all work with the ophthalmoscope or retinoscope it is understood that the observer's 

 eye is emmetropic, either by nature or by correction, and that his accommodation is sus- 

 pended. One may get a clear view of the retina without fulfilling these conditions, but one 

 cannot draw reliable optical conclusions. 



