1538 



THE AMERICAN MONTHLY 



[Nov 



raaintMiiKMl. The novelty, however, consists in t1ie sub- 

 staj^e condensing lens, e, which can be focnssed on the 

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

 The important point in tliis arrangement is tliat tlie focus 

 of the condenser is not distnrhcd whib> the object is being 

 focussed to or from the magnifying lens. To Hartsoeker 

 we are also indebted for v. 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- 



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- 



