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 



