test-objects: — nobert's test-plate. 179 



minuteness or their too close approximation, they may be enlarged 

 or separated by a deeper Eye-piece, provided that the Objective be 

 well corrected. But if, in such a case, the image be darkened or 

 blurred, so as to be rather deteriorated than improved, it may be 

 concluded that the Objective is of inferior quality, having either 

 an insufficient Angular Aperture, or being imperfectly corrected, or 

 both. 



in. All Object-glasses of less than 1 -5th inch focus may be classed 

 as high powers ; the focal lengths to which they are ordinarily con- 

 structed are l-6th, l-8th, l-12th, l-16th, l-20th, and l-25th of 

 an inch respectively ; the 1-1 6th and l-25th being made by 

 Messrs. Powell and Lealand only, and the l-20th by Messrs. Smith 

 and Beck : and the magnifying powers they are fitted to afford range 

 from about 320 to 1200 diameters with the shallower Eye-piece, 

 and from 480 to 1800 diameters with the deeper. By the use of 

 still deeper Eye-pieces, or by the Objective of l-50th inch recently 

 constructed by Messrs. Powell and Lealand, a power of 3500 or 

 more may be obtained ; but it is questionable whether anything 

 is really gained thereby. Moreover, as the 1-1 2th inch Objective 

 may have its angular aperture extended to 170°, the utmost limit 

 compatible with the reception of rays from any object (§ 14), 

 nothing is gained in this respect by a reduction of the focal dis- 

 tance; and the admirable l-12th now constructed by Mr. Ross 

 may be made to give an amplification equal to that of the l-25th 

 of Messrs. Powell and Lealand, with little if any inferiority in 

 defining and resolving powers. The use of this class of Objectives 

 is much more restricted than that of the preceding. They are not 

 suitable for the ordinary purposes of Scientific investigation ; and 

 their value chiefly lies in the power which they afford of tracing 

 out certain points of minute structure which the Objectives of 

 medium power may only doubtfully indicate, and of exhibiting 

 certain classes of very difficult striated or dotted objects which 

 these cannot resolve. Hence it is obvious that with regard to 

 Object-glasses of this class, Resolving power (coupled with Defining 

 power) is the highest requisite. Penetrating power and Flatness of 

 field being of secondary account ; and that the value of an Ob- 

 jective may here be fairly estimated by its angular aperture, pro- 

 vided that its aberrations be exactly corrected. Of Angular Aper- 

 ture and Definition very good tests are afforded by the lines 

 artificially ruled by M. Nobert, and by the more ' difficult ' species 

 of Diatomacese. What is known as Nobert's Test is a plate of 

 glass, on a small space of which, not exceeding one-fiftieth of an 

 inch in breadth, are ruled ten or more series of lines, forming as 

 many separate bands of equal breadth. In each of these bands the 

 lines are ruled at a certain known distance ; and the distances are 

 so adjusted in the successive bands, as to form a regularly diminish- 

 ing series, and thus to present a succession of tests of progressively 

 increasing difficulty. The distances of the lines differ on different 



n 2 



