The reader is requested to correct the following errata: 

 Page 21. Line 10 from bottom, strike out rest of paragraph 

 after "This is," and substitute: "This is found only in some 

 forms of the microscope." NOTE. The Jackson Model, shown 

 in Plate II, has a collar, and in most of the microscopes made 

 in France and Germany, the body moves through a collar. 



Page 36. Line 7 from bottom, for "Student" read "Pro- 

 fessional." 



Page 41. Line 4 from top, before the words "The Instru- 

 ment " insert " Other forms of." 



Page 62. Line 16 from bottom, for "effect" read "affect." 



Page 88. Line 12 from top, after the word "power," insert 

 "the want of." 



