APPARA TUS FOR SCIENTIFIC DEMONSTRA 7 ION 167 



third plano-convex lens, which converges the light, and over 

 whose face is the field of the instrument. A pillar at the side 

 supports the focussing power, a ring allowing various powers 

 to be used : and the same ring-fitting carries a second silvered 

 reflector, turning on an axis so as to throw the image at any 

 desired height on the screen. If a sheet of fine ground or 

 smoked glass be simply laid over the face of the large lens, a 

 diagram can readily be worked out, and is often better under- 



FiG. 89 



Fiii. 90 



stood when so worked out, than if projected complete. It will 

 be still more transparent if the glass is first rubbed with a 

 very little paraffin oil. 



Some opticians substitute a right-angled reflecting prism, 

 but the second reflection from a silvered mirror is practically 

 imperceptible except in white line diagrams drawn upon 

 really blackened glass. For such a prism is advisable, and is 

 always employed in Duboscq's form of the apparatus. 



An effective apparatus can be made very cheaply as in 



