196 MANAGEMENT OF THE MICROSCOPE. 



ral attributes whicli the Author thinks most important, he has 

 found no test more valuable and positive, as regards objectives 

 of from 4-lOths to l-5th inch focus, than Mr. Lealand's prepara- 

 tions of Muscular fibre (Fig. 326). In every case, the objective 

 should be tested v^ith the "deeper, as well as with the shallower 

 eye-piece ; and the effect of this will be a fair test of its merits. 

 Where markings are indistinguishable under a certain objective, 

 merely because of their minuteness or their too close approxima- 

 tion, 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 deteri- 

 orated than improved, it may be concluded that the objective is 

 of inferior quality, having either an insufiicient angular aperture, 

 or being imperfectly corrected, or both. 



m. AH object-glasses of less than l-5th of an inch focus, may 

 be classed as high powers ; the focal lengths to which they are 

 ordinarily constructed are l-6th, l-8th, l-12th, and l-16th of an 

 inch respectively ; and the magnifying powers they are fitted to 

 afford, range from about 320 to 850 diameters with the shallower 

 eye-piece, and from 480 to 1300 diameters with the deeper. By 

 the use of still deeper eye-pieces, a power of 2000 or more may 

 be easily obtained ; but nothing seems to be really gained by 

 such high amplification. Moreover, as the l-12th inch object- 

 glass may have its angular aperture extended to the utmost limits 

 compatible with the reception of rays from any object, it does 

 not seem that anything can be gained by a reduction of the focal 

 distance to the l-16th inch ; and the latter being a more difficult 

 combination, as well to construct as to use, both Opticians and 

 Microscopists have of late years found it advantageous to limit 

 themselves to the l-12th, which gives an amplification of about 

 650 diameters with the shallower eye-piece, and of about 1000 

 with the deeper. The use of this class of objectives is much 

 more restricted than that of the preceding. They are not em- 

 ployed 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 

 they cannot resolve. Hence it is obvious that, with regard to 

 object-glasses of this class, " resolving power" (coupled with " de- 

 fining power") is the highest requisite, "penetration" and "flat- 

 ness of field" being of secondary account; and that the value of 

 an objective may here be fairly estimated by its angular aperture, 

 provided that its al^errations be exactly corrected. Of angular 

 aperture and definition, very good tests are afforded by the lines 

 artificially ruled by M. ISTobert, and by the more " difficult" 

 species of Diatomacese. What is known as "Robert's Test" is 

 a plate of glass, on a small space of which, not exceeding a fiftieth 

 of an inch in breadth, are ruled ten or more series of lines, form- 



