194 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



2. Detection of Astigmatism. Determine the near point for a pin 

 held vertically and one held horizontally. Note whether the distances 

 correspond. If not, what are your conclusions. 



Draw two lines 5 cm. in length and intersecting at right angles. 

 Each right angle again bisect and each oblique angle again by other 

 lines, until a radiate figure is obtained. Interpose this figure at the 

 near point of accommodation. Note whether any of these lines appear 

 blurred. Explain. 



3. Measurement of Astigmatism. Carefully study the construc- 

 tion of the ophthalmometer (Helmholtz), and discuss the principle 

 involved in this method of detecting and measuring astigmatism. 



4. Visual Angle. The visual angle is the angle formed by the lines 

 drawn from the two extremities of an object through the nodal point 

 of the eye (optical center of the lens). It is situated 7 mm. behind the 

 cornea and 15 mm. in front of the retina. Since these lines traverse 

 the nodal point unrefracted, the size of the retinal image may be ob- 

 tained from this projection. Draw a diagram to show that, in order 

 to subtend at the same angle, objects must be made increasingly larger, 

 the farther they are removed from the retina. Thus, the letter A seen 

 clearly at 6 m., would have to be ten times as large at 60 m. in order 

 to be seen equally well. At a distance of 1 m. this letter should be 1 cm. 

 in height in order to be seen clearly by the emmetropic eye. 



5. Snellen's Test Types. Determine the normal acuity of vision 

 by distinguishing different letters subtending at an angle of 5 degrees. 

 If the distance indicated can be exceeded or cannot be reached, the 



D 



acuity of vision may be expressed as = -7. In this formula D stands 



u 



for the given distance, at which the angle of 5 degrees is subtended, and 

 d for the distance at which the letters can be recognized. 



Equip the ametropic person (myopic or hypermetropic) with lenses 

 of different refractive power until able to recognize these letters clearly. 



6. Ophthalmoscopic Examination of the Emmetropic Artificial Eye 

 by the Direct Method. Remove the projection-lens from the optical 

 lantern. Adjust the emmetropic artificial eye for far vision at zero, 

 and place it to the left of the lantern and on a level with your own eye. 

 With the right hand hold the ophthalmoscope close to your right eye 

 and about 30 cm. from the artificial eye. Make the visual axis of your 

 right eye coincide with that of the artificial eye by keeping your head 

 erect. Look through the opening in the mirror and throw the light 

 into the pupil of the artificial eye. Accommodate your own eye for an 

 imaginary object placed at some distance precisely behind the artificial 

 eye. Gradually move your head toward the artificial eye until the 

 mirror lies in the anterior principal focus of the latter, i. e., about 5 cm. 

 in front of the cornea of the artificial eye. 



When the fundus of the artificial eye has become visible, find the 

 optic disk. Draw a diagram showing the course of the rays of light. 

 In case the observer is myopic or hypermetropic, 'he must first cor- 



