1897J MICROSCOPICAL JOURNAL. 337 



lu 1672 Sir Isaac Newton suggested a reflecting micro- 

 scope of the form of a Herschelian telescope. It proba- 

 bly was never made. 



Leeuwenhoek's microscopes, constructed in 1673, are 

 remarkable more (>n account of the man who used them 

 than for their design, which was crude in the extreme. 

 It is indeed difficult to understand how the discoveries 

 he made could have been carried out with such rude 

 apparatus. 



In 1687 we find a microscope by Grindl very similar to 

 Fig. 5. The optical part, however, consisted of three 

 pairs of plano-convex lenses. 



In 1691 several new features appear. Fig. 6 shows a 

 screw-barrel compound microscope by Bonanui. The 

 slider placed between two plates pressed together by a 

 spiral spring, was made to approach or recede from the 

 objective by a screw. This simple arrangement, known 

 as the "screw barrel," played an important part in the 

 history of the microscope for upwards of 100 years. 



To Bonanni we are also indebted for a horizontal 

 microscope in 1691 (Fig. 7). This instrument is note- 

 worthy, first for the double support to the body. A 

 glance at Hooke's (Fig. 3) will convince anyone how 

 rickety the body must have been v/hen only held by its 

 focussing screw, so here we have a decided improvement. 

 Secondly, we have a rack, i, and pinion, h^ coarse adjust- 

 ment, in addition to the usual screw fine adjustment, 7n, 

 of that period. There is also an improvement in the 

 stage, and the last, and perhaps the most important nov- 

 elty, is the compound substage condenser, p, q. Hooke's 

 illuminating apparatus was, as we have seen, more suit- 

 able for opaque objects; this, on the other hand, is more 

 adapted for the illumination of transparent objects. We 

 now come to an excellent simple microscope by Hart- 

 soeker, in 1694 (Fig. 8). It will ]»e observed that the 

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



