338 



THE AMERICAN MONTHLY 



[Nov 



maintained. The novelty, however, consists in the sub- 

 stage condensing lens, e, which can be fociissed on the 

 object by screwing, /, into the screw focussing tube. 

 The important point in this arrangement is that the focus 

 of the condenser is not disturbed while the object is being 

 focussed to or from the magnifying lens. To Hartsoeker 

 we are also indebted for ft compressor. 



Wilson's screw- barrel, of 1702, then known as the 

 pocket microscope, was a popular form of simple micro- 

 scope in the 18th century ; it was very similar to Hart- 



^ / w^^-^ l^^^ jL 



soeker's, the main difference being that the substage 

 condensing lens had no separate focussing adjustment. 

 Culpeper subsequently mounted these microscopes on a 

 pillar rising from a flat folding tripod foot, a mirror and 

 condensing lens being attached ; he also added a com- 

 pound body to them. Later, in 1742, the Wilson screw 

 barrel was. mounted on a brass scroll fixed to a circular 

 wooden foot, to which was attached a concave mirror. 

 In this same year it is also stated that two diaphragms 

 were supplied with the ordinary hand Wilson screw- 

 barrel simple microscope, to fit in a cell close to the sub- 



