470 THE OUTGO OF ENERGY 



The right-hand image will disappear. 



Hold the needle about 100 cm. away and fix 

 some nearer object. The needle will appear 

 double. Close the left-hand hole. 



The left-hand image will disappear. 



Draw a diagram to explain these observations"; 

 Eemember that a separate image of the needle 

 will be formed by the rays passing through each 

 hole in the card. 



Dispersion Circles. Place a printed page 

 about two feet in front of one eye, and shut the 

 other eye. Observe the letters through a piece 

 of wire gauze held six inches in front of the eye. 



Either the wire or the letters can be distinctly 

 seen, but not both at once. If the letters are" 

 seen clearly, each wire will appear as a broad 

 indistinct line made up of superposed dispersion 

 circles and vice versa. 



Diameter of Circles of Dispersion. 1 . If the 

 eye be accommodated for objects at an infinite 

 distance (practically twelve metres or more), the 

 image of a near object will fall behind the 

 retina. The image will lie in the conjugate focus 

 of the object, and the position of the image can 

 be calculated by the formula for conjugate foci 

 (page 444). From this formula may be derived 





