354 



E. M. NELSON ON THE EVOLUTION OF THE MICROSCOPE. 



last, and perhaps the most important novelty, is the compound 

 substage condenser, p, q. Hooke's illuminating apparatus was, 

 as we have seen, more suitable for opaque objects ; this, on the 



Pig. 8. 



other hand, is more adapted for the illumination of transparent 

 objects. We now come to an excellent simple microscope by 

 Hartsoeker, in 1694 (Fig. 8). It will be observed that the 

 Bonanni screw-barrel focussing arrangement, c, d, is main- 

 tained. The novelty, however, consists in the substage con- 

 densing lens, e, which can be focussed on the object by screw- 

 ing, /, 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 a 

 compressor. 



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

 microscope, was a popular form of simple microscope in the 18th 

 century ; it was very similar to Hartsoeker's, the main diffe- 

 rence 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 

 compound 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 substage condenser, to reduce its aper- 

 ture when high powers were used. This is the earliest notice 

 of diaphragms for regulating the illumination. 



In the year, 1702, we find a crude form of simple microscope 

 by Mussenbroek. The only point of interest it possesses is to be 

 found in a sector of graduated diaphragm holes. The pur- 



" ll.nrv Baker " On the Microscope," 1st Edition, 1742. 



