808 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



be firmly fixed by a tightening screw, or can be turned aside, or 

 removed altogether, when it is desired to use the instrument as a 

 simple Microscope. The mirror is attached to the third pillar. 



Holmes's Class Microscope. — This instrument (Fig. 168), the de- 

 sign of Dr. 0. W. Holmes, of Boston, U.S.A., is substantially a 

 modified form of Beale's Demonstrating Microscope, except that the 

 tube is not in a horizontal but in a suitably inclined (fixed) position. 

 The wooden pillar on the left forms the handle for passing the instru- 

 ment round the class. The coarse adjustment is effected by sliding 

 the body through the outer split tube. The height of the instrument 

 is about 12 in., and the size of the base (on which it stands for 

 ordinary table use) 12 x 4 inches. 



A special peculiarity is in the fine adjustment, which is effected by 

 moving the stage. For this purpose tlae stage is suspended by its 

 lower edge to a metal hinge. A somewhat coarse-threaded screw 

 attached to the limb, and having a strong spiral spring coiled round 



Fig. 168. 



it, passes through the stage, and is acted upon by the nut with lever-arm 

 seen beneath. The lateral movement of this lever-arm in one direction 

 causes the stage (which is held between the spiral spring above and the 

 nut beneath), to tilt up from the hinged joint, the spring forcing the 

 stage back again when the lever is turned in the reverse direction. The 

 motion is therefore not strictly at right angles to the optic axis, but 

 for low-power work this is hardly of consequence. In lieu of a con- 



