VISION. 249 



ing lenses, with each one testing for a distinct reti- 

 nal image : 



* 1' D., 2 D., + 3 D., and+ 4 D. 

 With which one of the lenses is the clearest 

 image obtained? Are all of the images of equal 

 size? Explain, giving a figure.* 



(9) In hyperopia do the rays focus in front of, on, 

 or behind the retina? What direction do the rays 

 take after leaving the hyperopic eye from the illu- 

 minated retina? Are they parallel, divergent or 

 convergent ? 



Observation of the retina in a myopic eye. 

 Adjust the model for myopia, e. g., three dioptrics. 



(10) Are the retinal blood vessels distinct? 



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

 on emerging from the myopic eye, divergent, con- 

 vergent, or parallel? 



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

 eye be able to get a clear image of the retinal struc- 

 tures ? 



(13) In which case would a correcting lens be neces- 

 sary? Should one use a convex or a concave lens ; 

 and why ? 



*In all work with the ophthalmoscope or retinoscope it is under- 

 stood that the observer's eye is emmetropic, either by nature or by cor- 

 rection, and that his accommodation is suspended. One may get a clear 

 view of the retina without fulfilling these conditions, but one cannot 

 draw reliable optical conclusions. 



