MANAGEMENT OF THE MICROSCOPE. 169 



which brings out its markings satisfactorily will suit the requirements of 

 the ordinary working Microscopist, although it may not resolve difficult 

 Diatoms. In every case, the Objective should be tried with the B and 

 C as well as with the A eye-piece; and the effect of this substitution will 

 be a fair test of its merits. Where markings are undistinguishable under 

 a certain Objective, merely because of their 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 l-5th inch focus may be classed 

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

 being l-6th, l-8th, l-10th, l-12th, l-16th, l-20th, l-25th, l-40th, and 

 l-50th of an inch respectively; the l-12th, l-16th, l-25th, and l-50th 

 being made by Messrs. Powell and Lealand, and the l-10th, l-20th, and 

 l-40th by Messrs. Beck. The magnifying powers which Objectives from 

 l-6th to l-25th inch focus are fitted to afford, range from about 320 to 

 1250 diameters with the shallower Eye-piece, and from 480 to 1850 dia- 

 meters with the deeper: but by the use of still deeper Eye-pieces, or by 

 the Objective of l-50th inch, or the l-80th recently constructed by 

 Messrs. Powell and Lealand, a power of 4000 or more may be obtained. 

 It is seldom, however, that anything is really gained thereby. The in- 

 troduction of immersion-lenses ( 19) has considerably increased the 

 utility of what may be called moderately high powers, such as l-8th, 

 l-10th, and l-12th. These, if really good, can be used when necessary 

 with deep Eye-pieces; and very little of scientific importance that is be- 

 yond their reach has yet been seen by higher Objectives, though the lat- 

 ter have, no doubt, special value in certain circumstances when skilfully 

 employed. With these and higher powers not intended for exclusive 

 use upon 'vexatious' Diatoms, the Angle of aperture should be so pro- 

 portioned to focal length, as not to sacrifice the ' definition ' and ' penetra- 

 tion ' required to show the internal organs of small Eotifers, large Infu- 

 soria, minute Worms, etc. An Objective that will show surfaces only, 

 may be broadly stated to be of little use for Biological investigation. 

 Dry-front l-8ths or l-12ths with an aperture closely approaching 170, 

 are of very limited utility, from want of penetration, and from focussing 

 extremely close to their objects; while with 30 to 40 less aperture and 

 good corrections, they are much more serviceable, losing very little (as 

 already shown, 158, iv.) in 'resolving' power, and gaining much in 

 working distance and penetration. 



160. For Eesolving power, the best tests are afforded by the lines ar- 

 tificially ruled by M. Nobert, and by the more ' difficult ' Diatoms. 

 What is known as Noberfs Test is a plate of glass, on a small space of 

 which, not exceeding one-fiftieth of an inch in breadth, are ruled from 

 ten to nineteen 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 regular diminishing series, and thus to present a succession of 

 tests of progressively increasing difficulty. The distances of the lines 

 differ on different plates; all the bands in some series being resolvable 

 tinder a good Objective of l-4th inch focus, whilst the closest bands in 

 others long defied the resolving power of l-12th inch Objectives of large 



