MICROSCOPIC OBJECTS. 105 



I place those which operate out of focus and tell us what 

 the defects of an instrument are ; — in the second, those 

 which, if exhibited by a microscope, assure us that it 

 possesses certain good quahties. The first division, as 

 artificial stars, enamel dial-plate, wire-gauze, &c.* — ^which 

 informs us of the state of their aberration, achromatism, 

 centering, adjustment, curves, &c. — I shall pass over, as 

 many persons are not disposed to enter into a scientific 

 scrutiny concerning the causes of then" demerits, and 

 because they are more apphcable to engiscopes or com- 

 pound microscopes, than to single and compound mag- 

 nifiers ; and shall content myself by giving some simple 

 means of determining effectiveness by means of the 

 second division. '' 



" Before describing the lined test-objects individually, 

 I should notice that they differ much in the facility with 

 which they are resolved. Some are just made out by an 

 ordinary microscope ; while others require the most per- 

 fect instrument and precipe tact in the management of 

 the illumination. It will be proper, therefore, to divide 

 them into classes ; the first containing such objects as 

 are most easily resolved ; the second, such as require an 

 instrument having very clear and distmct vision ; and the 

 last, the most difficult to which the powers of a microscope 

 can be subjected, and requiring the most rigorous per- 

 fection in every respect for their resolution. When an 



* For a particular account of these objects see Dr. Goring's 

 Memoir "on the Exact Method of &c." in the Microscojnc Illustra- 

 tions, 3rd edition, page 264. 



