Page:The American Cyclopædia (1879) Volume XI.djvu/533

 MICROSCOPE 515 The proportion between the two images so seen will be the focal length required. The panes of glass in a window, or courses of bricks in a wall, are convenient objects for this pur- pose." The comparative focal lengths of two lenses, or sets of lenses, may be determined by holding them at the same distance from the eye and estimating the size of the image formed by each of the same object ; thus, if one lens forms the image half the size of the other, lineal measure, its focal length is half that of the other. The same method applies to eye pieces. For a history of the earlier forms of the compound microscope, the reader may consult the elaborate works of Quekett and Harting and the older works of Adams and Baker. Essentially it consists of two parts, the object glass and the eye piece. The for- mer is now made by A a combination, usual- ly, of three sets of achromatic doublets, arranged to give the greatest freedom from spherical and chro- matic aberration ; the latter, of two plano- convex lenses, with the plane sides to the eye, the lens nearest to the object, or "field lens," being almost exactly double the fo- cal length of the eye lens, and the distance between them a little more than the focal length of the field lens ; the ratio is va- ried somewhat by dif- ferent makers. In fig. 1, the object placed at P, on the stage T, is illuminated by con- verging rays, 5, 5, 5, &, reflected from the FIG. l. mirror S. At L is the compound achro- matic object glass. C is the field lens, and A the eye lens of the eye piece. With the eye lens one views the image of the object, P', formed by the object glass. The eye piece thus formed is termed a " negative eye piece," or the " Huygenean." The eye piece of Kell- ner is a decided improvement ; it is termed "orthoscopic," and the eye lens is achromatic or nearly so ; these eye pieces are supplied by the Messrs. Grunow of New York, with their best instruments ; the field of view is large, free from distortion, and well defined through- out the whole extent. The orthoscopic eye piece supplied by Mr. Charles A. Spencer of Canastota, N. Y., and more recently as im- proved by R. B. Tolles of Boston, has both eye and field lens achromatic, and is exceedingly perfect; it is, however, more expensive than the Kellner eye piece. Mr. Tolles has intro- duced a 'solid, orthoscopic, negative eye piece, of remarkable clearness and definition through- out, especially fitted for micrometric use, the engraved scale being cemented in the body of the solid eye piece, and perfectly protected from all dust or interference with definition, so noticeable in the use of the eye-piece micro- meter in the ordinary way. Mr. Tolles has also introduced what is termed an amplifier, being an achromatic concave of peculiar con- struction, which is introduced within the body of the microscope by means of an adapter. The corrections of the objective are not in the least disturbed by this arrangement, but the power is doubled. A low eye piece thus gives as much amplification as a higher one, and with the very great advantage of almost perfect flat- ness of field. The object glasses, or " objec- tives " as they are now very commonly termed, derive their denominations, 1 inch, inch, inch, &c., from the fact that the combined sets of lenses give a magnifying power the same as a single lens of the same name. Thus, a ^object glass should give the same amplifi- cation as though a single lens of inch was used in its place. This term does not refer at all to the working distance, for, as is the case with doublets, the working distance with all powers higher than the inch is considerably less than that of the equivalent single lens ; it will be apparent that for any given focus the working distance will, in general, be diminished by an increased angle of aperture ; a ^ of 90 will have in this respect a very great advan- tage over a J of 140. As regards the merits of the large angle objectives, there are various opinions. Dr. Carpenter is decided in his con- demnation, considering that depth of penetra- tion cannot be had at the same time with en- larged angle. The skill of the first opticians, Spencer, Tolles, and Wales in America, Ross, Powell and Lealand, and Smith, Beck, and Beck in England, have proved the contrary. Nor is Sir David Brewster's assertion true of large angle objectives, that they give a distorted view. The definition, clearness, and perfect- ness of vision with Powell and Lealand's -fa, having an angle of 176, when employed with a low eye piece so as to give the same ampli- fication as a with a higher, is greatly supe- rior to that of the. The chief advantage of the small angle has been considered to be the sort of general view it would give of the whole of a minute object ; the working distance being so great that the minute elevations and depressions in the object itself, being but a very small fraction of the whole distance, would not perceptibly affect the focus. In many respects this might be a desirable qual- ity ; but the inicroscopist would quite as often find it a source of error in his interpretation of what he might observe, and he will find it safer to decide as to the elevations and de- pressions of an object by the removal and ap- proach required to be given to the object glass,