8 PRACTICAL TKEATISE ON 



as because that light itself is a heterogeneous mixture of dif- 

 ferently refrangible rays." Having constructed a telescope on 

 the reflecting principle, Newton was soon led to apply the 

 same principles to the microscope, and we find that m^ the 

 year 1672 he invented the first compound reflecting micro- 

 scope, since so greatly improved by Amici, Cuthbert, and 

 Dr. Goring. Newton also suggested that the compound 

 refracting microscope would be rendered more perfect "if 

 the object to be viewed were illuminated in a darkened 

 room by light of any convenient colour not too much com- 

 pounded;" in fact, monochromatic hght. 



In the year 1698, Philip Bonnani, in his work entitled 

 Observationes circa viventia, quae in Rebus non viventibus repe- 

 riunfur, describes a compound microscope in use by him. 

 This microscope, which is represented by fig. 4, was placed 



Fig. 4. 



on a stand in the horizontal position, and was provided 

 with a stage for the objects ; and, with a coarse and fine 

 adjustment to the compound body, the former was obtained 

 by means of a rack and pinion, which moved the entire 

 frame-work, supporting the compound body, whilst the latter 

 was efiected by a screw in the end of the body itself near 

 to the object glass; and to steady the opposite end of the 



