430 THE OUTGO OF ENERGY 



cles. Similar blurring by dispersion circles is 

 caused by the rays which pass through the outer 

 parts of a lens coining to a focus sooner than 

 the axial rays. 



Myopia. In the normal eye at rest parallel 

 rays are brought to a focus upon the retina. In 

 the myopic eye parallel rays, and even rays to a 

 certain degree divergent, are brought to a focus 

 in the vitreous, whence they fall in dispersion 

 circles on the retina. The most common cause 

 of myopia is the abnormal length of the antero- 

 posterior diameter of the eye. The defect can be 

 remedied by placing a concave lens before the 

 eye. The entering rays are thereby rendered 

 divergent, or their divergence is increased, so that 

 their focus is displaced backwards towards the 

 retina. The degree of the myopia is measured 

 by the strength of the concave lens which, placed 

 before the eye, will bring the principal focus 

 exactly to the retina. 



Let parallel rays pass through the convex lens 

 of 10 cm. (4 inch) focal distance placed against 

 the window of the optical box. Find the prin- 

 cipal focus and then move the screen 2.5 cm. 

 farther from the lens. 



The image will be blurred. The screen will 

 intersect the rays diverging from the focal point. 



Hold the weak concave lens, marked 2, in 



