194 



CLASS DEMONSTRATIONS 



[CH. VII 



tube, and for roughly focusing the sliding motion suffices ; for fine 

 adjustment, the sheath is made to turn on a fine screw thread on a 

 cylindrical tube, which serves also as a socket carrier for the stage. 

 The compound microscope is here reduced to the simplest form I 

 have met with to be a really servicable instrument for the purpose 

 in view; and the mechanism is of thoroughly substantial character. 

 I commend this model to the notice of our opticians." 



Fig. 149. Dissecting microscope. This is convenient for demonstrations 

 of rather large objects. If they are transparent then the mirror is used. If 

 the objects are opaque they must be lighted by a mirror above the stage or by a 

 bull's eye condenser. In this one the focusing is done by a rack and pinion. 

 (Cut loaned by the Bausch & Lomb Optical Co.) 



Since its introduction by Tolles many opticians have produced 

 excellent demonstration microscopes of this type, but most of them 

 have not preserved a special mechanism for fine adjustment. With 

 it one can demonstrate with an objective of 6 mm. satisfactorily. It 



