160 EXPERIMENTAL PHYSIOLOGY. 



parallel by a divergent lens, and the strength of the lens which 

 one must use for this gives a measure of how much the rays differed 

 in direction from the parallel paths which they should have taken. 

 If the eye is LONG-SIGHTED the condition is reversed. The 

 retina is farther forward than the principal focus of the eye, the 

 rays diverge too sharply and after refraction they still diverge to 

 some extent. The observer to see clearly must in this case use a 

 conyergent lens and, as before, the strength of the lens gives a meas- 

 ure of how different the eye is from the normal. This is known as 

 the DIRECT METHOD of using the ophthalmoscope. The part of the 

 retina which one can see at one time in this way is a small one and 

 for that reason, and sometimes too because it is difficult for the 

 observer, if he is inexperienced, to relax his accommodation com- 

 pletely, the INDIRECT METHOD is often used (Fig. 44, B.). The 

 arrangement of light and mirror is on the same plan as before, but 

 in this case a convex lens is held immediately in front of the subject's 

 eye. This focusses the rays emerging parallel from the eye and 

 forms an inverted real image of the bright spot of the fundus, at 

 the principal focal plane of the lens. It is this image at which the 

 observer looks. 



Experiment 63. The use of the ophthalmoscope for the examina- 

 tion of the human eye is not suitable as a laboratory experiment 

 since a beginner to have success needs both time and quiet. 

 Each student is expected, however, at some time during this 

 part of the course, to procure an instrument, of which there are 

 several available in the supply room, and to practise with it 

 until he can see for himself the interior of a normal eye. Using 

 both the direct and indirect methods, work at the experiment 

 until you can see many or all of the details of the interior surface 

 of the eyeball described below (p. 147). Before doing this it is 

 wiser to practise a little in the laboratory, using the instrument 

 according to both methods to examine the eye of a rabbit, the 

 pupil of which has been dilated with atropine. 

 Direct Method. Place the light and ophthalmoscope, as 

 shown in Fig. 44, and look from behind the mirror through 

 the hole. Start with it about eighteen inches away from the 

 observed eye and move it slowly about until the pupil is seen 

 as a bright red spot. Now move the mirror gradually nearer, 



