490 THE OUTGO OF ENERGY 



glass, silvered, pierced at its middle point with 

 an aperture of about 2.5 mm., pivoted to turn to 

 either side ; (2) two rotating disks carrying a 

 series of concave and convex lenses in front of 

 the aperture. 



The silvered mirror reflects more light than 

 the mirror of transparent glass. Further, it 

 allows the lamp to be placed at the side of the eye 

 to be examined, and at any required distance from 

 the mirror. The turning of the mirror upon a 

 pivot permits the more or less oblique incident 

 rays to be thrown into the eye without tilting 

 the disks carrying the lenses, and thus rendering 

 the lenses astigmatic by placing them at an 

 angle to the optical axis which passes from the 

 subject's retina through the aperture of the mir- 

 ror and through the lens behind the aperture 

 into the observer's eye. 



The disks may be used singly or in combi- 

 nation. A series of concave lenses (marked ) 

 from 1 D to 24 D, and a series of convex lenses 

 (marked +) from 1 D to 23 D, are thus secured. 



DIRECT METHOD 



Emmetropia. 1. Eemove from the lantern 

 the tubes holding the projecting lens. Place the 

 ground glass screen before the condenser. See 



