486 THE OUTGO OF ENERGY 



axis, and thus intercepts the rays from the source 

 of light to the subject's eye. The interior of the 

 eye is therefore not illuminated, and the pupil 

 remains dark. 



3. Three millimetres behind the principal 

 point of each of the two reduced eyes draw a 

 diaphragm (iris) with an aperture (pupil) four 

 millimetres in diameter. Assume that the sub- 

 ject's eye is accommodated for the pupil of the 

 observer's eye. 



Note that a dark image of the pupil of the ob- 

 server's eye will be formed on the retina of the 

 subject's eye. The rays reflected from this image 

 will form a second dark image which will exactly 

 coincide with the pupil of the observer's eye. 

 Thus the observer will see only the reflection of 

 his own black pupil in the subject's eye. 



4. Throw light into the subject's pupil from a 

 lamp held as near the observer's eye as possible. 

 The subject should not look at either the observer 

 or the light, and his eye should be accommodated 

 for a distance much less or much greater than 

 that of the observer or the light. 



Part of the pupil will appear red. It has been 

 shown in Constructions 2 and 3 that the pupil or- 

 dinarily appears black. When, however, a part of 

 the image of the light on the retina of the subject 

 coincides with that of the pupil of the observer, 



