E. M. NELSON ON EVOLUTION OF THE MICKOSCOPE. 



101 



the microscope, (one would have thought that they would have 

 been more conveniently placed behind the pillar,) regulate the 

 movements of the stage ; one causes it to move vertically up and 

 down for focussing, another makes it advance to and from the 

 pillar — this would now be called the vertical movement of a 

 mechanical stage ; the third and lowest nut rotates the stage. 

 Attention must be given to this last point because this is not 



Fig. 12. 



what is now known as a rotary stage, but it is a transverse move- 

 ment in arc, a device not altogether unknown in recent times. 



Artificial illumination is provided for opaque objects by means 

 of a candle, a concave mirror, and bidl's eye ; this plan, however, 

 cannot claim originality, as it had been previously employed by 

 Hooke (in Part 1, Fig. 3, Yol. 6, Ser. 2, p. 351, the concave 

 mirror is not shown). The two points in the history of the 

 evolution of the microscope presented by this instrument are 

 the mechanical stage and mirror. 



