31 



vulcanite mountings are discarded for those made 

 of metal, especially German silver, although many 

 are used of pure silver and some even of gold. 



Reading Glass. - - When magnifying lenses 

 reach a diameter of 2 inches or more they are 

 usually termed Reading Glasses and are then pro- 

 vided with a handle. They are used to enlarge 

 small printed matter, or to determine detail in 

 engravings and photographs. 



Bruecke Lens.- -This is a combination of lenses 

 named after the inventor which, while at one time 

 a popular construction went almost into disuse, 

 but in recent years has been found to have valu- 

 able properties and is now very generally used. A 

 combination of achromatic lenses is superposed by 

 an achromatic concave lens and gives a very satis- 

 factory image. Much higher power can be ob- 

 tained than would be possible in any other form of 

 simple microscope. While it has the disadvantage 

 of a small field, this is more than compensated for 

 by a much larger focal distance than would be the 

 case in the ordinary forms of simple or compound 

 microscope, thus making dissection possible during 

 observation and a further advantage over the 

 compound microscope, in that the image is erect 

 as in the simple form and not reversed as in the 

 compound. A change of powers is made conven- 

 ient either by changing the lenses or varying the 

 position of the eye lens. 



