196 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



clear. If the artificial eye is hypermetropic, use convex lenses instead 

 of concave. The focal power of the lens which is needed in order to 

 be able to see the fundus of the eye of the subject clearly, is the lens 

 needed to overcome the ametropia of this eye. In prescribing glasses 

 for patients the strength of the lens is somewhat exaggerated, i. e., we 

 prescribe in hypermetropia the strongest convex lens with which the 

 observer is still able to see the details of the fundus, and in myopia the 

 weakest concave lens. Why? Draw diagrams to show the course of 

 the rays emitted by myopic and hypermetropic eyes. Also indicate 

 how these conditions may be corrected by the interposition of suitable 

 lenses. 



8. Ophthalmoscopic Examination of the Artificial Eye by the In- 

 direct Method. Place a convex lens of 5 diopters behind the opening 

 in the mirror of the ophthalmoscope, and hold the latter about 30 cm. 

 in front of the artificial eye. Interpose at a distance of 5 cm. from the 

 cornea of the artificial eye a +20 lens, held between the thumb and 

 forefinger of the left hand. The rays which issue from the artificial eye 

 are focalized in space after they have passed through the +20 lens. 

 This image you study with the aid of the +5 lens. Draw a diagram to 

 show the course of the rays emitted by this eye. 



9. Skiascopy or the Shadow Test. Hold the plane mirror of the 

 ophthalmoscope in front of your eye and at a distance of 1 m. from the 

 emmetropic artificial eye. Throw a beam of light upon this eye. That 

 portion of it which falls upon the pupil is condensed on the retina and 

 forms here an area of light which moves in accordance with the move- 

 ment of the mirror. Consequently, the same condition is obtained here 

 as when light is reflected against a wall by means of a mirror. Draw 

 a diagram to show the real movement of the light upon the retina, 

 note that its movement, as seen through the pupil, is the same as that 

 of the light on the face. 



Render the artificial eye myopic by drawing its halves farther apart. 

 Place the mirror at a distance of 1 m. from the eye and tilt it gently. 

 Note that the pupillary reflection now moves in a direction opposite to 

 that of the mirror. Move closer to the eye until a point has been reached 

 at which it is impossible to tell which way the light moves. Move still 

 closer to the eye and observe that the light now moves with the rota- 

 tion of the mirror. It is to be remembered that the myopic eye con- 

 verges the rays leaving it. The point in space at which they intersect 

 is the point of reversal. Obviously, distally to this point the light must 

 move against, and inside this point, with the rotation of the mirror. 

 Hence, skiascopy is simply an accurate method of determining this 

 point of reversal. Draw a diagram illustrating the reflection in myopia. 



Render the artificial eye hypermetropic by bringing its halves closer 

 together. Place your eye at a distance of 1 m. from the artificial eye. 

 Move back and forth and note that the movement of the pupillary light 

 is with the rotation of the mirror, no matter what position you occupy. 

 This is due to the fact that the hypermetropic eye does not form a 



