488 THE OUTGO OF ENERGY 



2. Repeat Construction 1, but bring the light 

 nearer the observer's eye. 



Diminishing the angle between the axis of the 

 observer's eye and the axis of the illuminating 

 pencil increases the length of the cone formed by 

 the intersection of the illuminating pencil and 

 the pencil to the observer's eye. Thus the ob- 

 server sees a larger cross-section of the lens and 

 vitreous, and sees farther into the eye. 



3. Repeat Construction 1, but place the ob- 

 server's nodal point in the axis of the illuminat- 

 ing pencil. 



The point of the cone will reach the retina. 

 The light reflected will emerge from the emme- 

 tropic eye in parallel rays which will enter the 

 observer's eye and form upon his retina an image 

 of the illuminated area TD! the subject's retina. 



Influence of Size of Pupil. Repeat Construc- 

 tion 1 of the preceding section, but enlarge the 

 diameter of the pupil to eight millimetres. 



The visible portion of the interior of the eye is 

 greater with a large pupil than with a small one. 



Influence of Nearness to Pupil. Repeat Con- 

 struction 1. but draw the observer's eye nearer the 

 subject's eye. 



Note that rays from a larger portion of the 



article on the Ophthalmoscope printed in the first edition of 

 Wood's Reference Handbook of the Medical Sciences, 



