460 MANIPULATION. 



glasses. The objects which have been chosen by the author, 

 as illustrations of the definition of microscopes as now con- 

 structed, are, with few additions, of the same nature as those 

 employed by Mr. Pritchard, in 1832, and published by him in 

 the Microscopic Cabinet. Plate 12 of that work, although at 

 the date of its publication one of the finest specimens of the 

 kind that had ever been executed, when contrasted with 

 plates 6, 7, 8 of the present work, will show, better than 

 words can express, the rapid improvements that have been 

 made in the construction of the object-glass ; it will, however, 

 be readily seen that the magnifying powers employed in the 

 latter instance were much greater than those used by Mr. 

 Pritchard, consequently a greater amount of detail ought to 

 be shown by the one than by the other ; this remark would 

 more especially relate to fig. 4a in plate 6, and figs. 5, 6, 7, 

 in plate 7, where the Knear magnifying power used was 1,200. 

 Before enumerating the test objects, of which a full explanation 

 will be presently given, it will be as well, in this place, to 

 allude to the means employed to ascertain the defects that may 

 be present in any achromatic object-glass; these defects, as 

 before stated, are chiefly spherical and chromatic aberration, 

 caused by bad centering, or adjustment, achromatism, and 

 want of angular aperture. AU these, with the exception of 

 the last, are so difficult of detection, that very few persons, 

 except those constantly engaged in the manufacture and 

 testing of object-glasses, can be said to be capable of discover- 

 ing them. The method usually adopted to ascertain the 

 presence of these defects, is a minute globule of mercury, 

 spread upon a black ground; this is known by the name of 

 " artificial star," and presents a minute point of light. Dr. 

 Goring alludes to the employment of an enamel dial-plate and 

 wire gauze for the same purpose, but only the mercury is now 

 used. Very minute globules of this metal, spread upon a 

 blackened surface, are viewed as opaque objects, being 

 illuminated by ordinary day-light from a window, or by the 

 light of an argand lamp, thrown on them by a condensing 

 lens ; when one of the globules is in focus of a single lens 

 object-glass, a strong mistiness surrounds the miniature image 



