Microscopes of Powell, Boss, and Smith. By E. M. Nelson. 431 



fine adjustment and the stage remain as before ; the lower part of the 

 stage is better seen in this figure ; the rotating diaphragm is attached 

 to a plate, which slides in grooves below the stage ; it is removable, 

 and an achromatic condenser can be inserted in its place. The large 

 lower milled head is the clamp of the compass joint. The foot is 

 circular and capable of rotation, so that the greatest amount of 

 stability can be secured when the instrument is used in either an in- 

 clined or a vertical position. This excellent idea, which was first intro- 

 duced by Cuff in 1705, is still carried out. 



Fig. 114. 



Andrew Ross's next model was constructed upon a totally dif- 

 ferent plan, as we can see from fig. 114, which is copied, from a very 

 rare book, the London Physiological Journal for Dec. 1813. This 

 figure is so clear, and the model so well known, that a description is 

 superfluous ; the following improvements may however be pointed 

 out. The hanging of the instrument between two supports is far 

 preferable to the former method of fixing it upon the top of a com- 

 pass joint ; this, as we have seen, was the invention of George Jackson ; 

 its centre of gravity is lowered, and its poise is in every way better. 



2 g 2 



