TEST OBJECTS. 459 



the angle of aperture of the object-glass; Dr. Goring com- 

 municated the result of his labours to the scientific public in 

 the journal of the Eoyal Institution, and, in 1829, gave a 

 more practical account of the same in the Microscopic Illus- 

 trations of Mr. Andrew Pritchard, to which the reader is 

 referred. Dr. Goring divided test objects into two classes. 

 In the first he placed those which it was necessary to examine 

 out of focus, such as minute globules of mercury, called 

 " artificial stars," by these the aberrations, achromatism, 

 centering, &c., were ascertained ; and in the second the lined 

 or other objects which, according as they were well or ill 

 defined, afforded sufficient evidence of the merits of the 

 instrument. 



Mr. Pritchard, following Dr. Goring, divides test objects 

 into two classes, viz., into those that are tests of the pene- 

 trating and those of the defining power of the instrument; 

 the words defining power, according to Dr. Goring, meaning 

 nothing more than a destitution of both kinds of aberration, 

 considered independently of the aperture of the microscope, 

 and that of penetrating power merely a large angle of aper- 

 ture.* Although this distinction of tests was necessary at the 

 time when Dr. Goring wrote, such is not the case now ; the 

 wonderful improvements that have taken place in the con- 

 struction of achromatic object-glasses within the last fifteen 

 years, whereby aU the errors of aberration, centering, and 

 achromatism have been so correctly balanced, have rendered 

 the microscope the most perfect and efficient instrument 

 " ever yet bestowed by art upon the investigator of nature." 

 The division of tests, therefore, into those of penetration and 

 definition is no longer needed, and the words defining power 

 and definition will be the only expressions employed to denote 

 the good or bad qualities of any microscope, for in order 

 that an object-glass may show the tests enumerated by Mr. 

 Pritchard, under the head of " Penetration," in a perfect 

 manner, its definition or defining power must be of the first 

 order, and these terms are, therefore, sufficiently explanatory 

 of the principal point to be attended to in testing two or more 

 * Microscopic Cabinet, p. 173. 



