PEACTICAL TREATISE ON 



power was increased by draw tubes from 40 to 140 times. 

 The latter, S. Campani of Bologna, was also a maker of 

 telescopes and microscopes, and a successful rival of the former, 

 his instrument was somewhat similar to that made by Divini, 

 being on the principle of an inverted telescope. Campani'a 

 lenses are said to have been worked on 

 a turn-tool, and not moulded. In 1672, 

 we find that S. P. Salvetti made micro- 

 scopes in imitation of those of Divini 

 and Campani, which were found to far 

 exceed those of the above-mentioned 

 artists in their magnifying and defining 

 powers ; but we are not told in what 

 points of construction these instru- 

 ments differed from those of his prede- 

 cessors. 



In the year 1673, the name of the 

 immortal Leeuwenhoek first appears 

 in the Philosophical Transactions of 

 this country, as a discoverer of nu- 

 merous wonders by aid of the micro- 

 scope; his instruments, which were composed of single lenses, 

 are said to have been greatly superior to all that had been pre- 

 viously made. According to Baker, 

 they were also remarkable for their 

 simpUcity, each one consisting of a 

 single lens set between two plates of 

 silver, perforated with a small hole, 

 with a moveable pin before it, to 

 place the object on and adjust it to 

 the eye of the beholder. "It has 

 been stated by many authors," says 

 Baker ( On Microscopes, vol. ii.), « that 

 the magnifiers used by Leeuwenhoek 

 were globules or spheres of glass, like 

 those invented by Hooke, but such 

 is not the case ; he assures us that in 

 the cabinet of the twenty-six micro- 



Fig. 2. 



