THE MICROSCOPE. 21 



to pursue original investigations with a general knowl- 

 edge of what has been accomplished by others. To this 

 end a comprehensive view of the necessary instruments 

 and details of the art, or what the Germans call technol- 

 ogy, is first given, and then a brief account of the appli- 

 cation of the microscope to various branches of science, 

 especially considering the needs of physicians and stu- 

 dents of medicine. 



CHAPTER II. 



THE MICROSCOPE. 



The Simple Microscope. The magnifying power of a 

 glass lens (from lens, a lentil ; because made in the shape 

 of its seeds) was doubtless known to the ancients, but only 

 in modern times has it been applied in scientific research. 



The forms of lenses generally used are the double convex, 

 with two convex faces ; piano convex, with one face flat 

 and the other convex ; double concave, with two concave 

 faces ; plano-concave, with one flat and one concave face ; 

 and the meniscus, with a concave and a convex face. 



In the early part of the seventeenth century very mi- 

 nute lenses were used, and even small spherules of glass. 

 Many of the great discoveries of that period were made 

 by these means. A narrow strip of glass was softened in 

 the flame of a spirit-lamp and drawn to a thread, on the 

 end of which a globule was melted and placed in a thin 

 folded plate of brass, perforated so as to admit the light. 

 Some of these globules were so small as to magnify sev- 

 eral hundred diameters. Of course, they were inconve- 

 nient to use, and larger lenses, ground on a proper tool, 

 were more common. 



The magnifying power of lenses depends on a few simple 



