516 MICROSCOPE in order to bring successively these elevations and depressions into distinct view. In minute and elaborate investigations the high angle ob- jectives are the most trustworthy. Moreover, the skill of the opticians named has enabled them to increase the angle without diminishing so. very much the working distance. The -^ of Mr. Ross, and we believe the T V of Powell and Lealand, will work through glass y^ of an inch thick. In speaking of the objectives of the prominent makers, we do not desire to indicate any order of precedence ; they are all excellent, and all have peculiarities of their own. We must, however, be permitted to notice a little more fully the American artists. The oldest optician, and the one who has been most known in connection with the microscope in the Uni- ted States, is Mr. Charles A. Spencer of Ca- nastota, N. Y. The object glasses furnished by him, particularly the later ones, are of the highest order; they range from 3 in. to ^ in. By many of our most experienced mi- croscopists they are considered superior to the best objectives of the London opticians ; they are certainly equal to them. Mr. Spencer, in the earlier days of high angle objectives, no doubt surpassed in this respect all the English opticians ; his rare skill and nice manipulation enabled him to perform wonderful feats in this direction, far in advance of anything be- fore accomplished. Mr. Robert B. Tolles, for some time connected with Spencer, but now by himself at Boston, has devoted himself to the perfection of the achromatic objectives with enthusiastic zeal and unparalleled success. His recent objectives are quite equal in de- fining and penetrating power to the very best of the London opticians, both with central and oblique illumination, and greatly superior to them in the latter case, as regards chromatic aberration ; this is true also of Mr. Spencer's objectives. Mr. Tolles's objectives range from 3 in. to T^J- inch; the latter objective is a marvel of optical art. Messrs. J. and W. Grunow of New York have sent out some very fine objectives, ranging from 2 in. to T V in. They have not attempted generally so high angles as Spencer and Tolles, but have devoted great attention to the mechanical arrangement and efficiency of their stands, and the accessory apparatus. Mr. William Wales came to this country in 1862, and settled at Fort Lee, N. J. His objectives are known the world over for their excellence; and among Tolles, Wales, and Powell and Lealand it is now impossible to assign any superiority in the performance of their lenses. The supe- rior workmanship and elegant form of the stands made by Mr. Joseph Zentmayer of Philadelphia are perhaps not equalled, certain- ly not surpassed, by the production of any other maker. (See fig. 2.) Among other im- provements given by this artist to the work- ing microscopist we would specially mention the remarkably thin and steady stage of his " grand American microscope," his invaluable glass stage, and his mechanical finger for pick- ing up and arranging diatoms. The latter invention, described in the "American Journal of Science " for May, 1870, is an improvement on the invention of the distinguished American microscopist Prof. II. L. Smith, described in the same journal for May, 1866. Of English opticians, the name of Andrew Ross has al- ways been placed foremost, being connected with the greatest improvement of the objec- tive, without which the higher powers of large angle would be almost valueless; we allude to the adjustment for cover. First of all the FIG. 2. opticians Mr. Ross made his objectives so per- fectly corrected for spherical and chromatic aberration, that a new source of difficulty, apparently almost insurmountable, presented itself. He found that these aberrations, so nicely balanced, were disturbed by each vary- ing thickness of the thin covering glass over the object. The expedient he devised to rem- edy this, was to alter the distance between the first set and the two posterior sets of achro- matics composing the objective, by means of a delicate screw collar. This grand and capi- tal improvement, for which Mr. Ross deserves the rank assigned him, has been adopted by all
