Page:The New International Encyclopædia 1st ed. v. 13.djvu/500

* MICROSCOPE. 453 MICROSCOPE. During the later Middle Ages such simple lenses came more ami more into use, especially as aids to the eye in urdimuy vision, as spec- tacles. A spectacleniakcr of .li<ldelburg, Hol- land, Zacharias Janssen, undoubtedly was the first to build a compound miscroscope, and about 15S0 constructed such an instrument and presented it to Charles Albert, Archduke of Austria. It was nearly six feet long, su[)portod upon brass dolphins on an ebony board. It con- tained only two lenses. Kobcrt llooke ( l(i3i)- 1703), secretary of tlic Royal Society, made many improvements in the construction and use of the microscope, and Divini in 1(568 improved the instrument by using two plano-convex lenses as an eyepiece (see below). In l(i.S(i t'ampani improved the form of the instrument and intro- duced the use of a screw for proper focusing. Nevertheless the development of the microscope took a difTerent direction, on account of the seri- ous ditlicullies with ahcrratiiin (q.v.) in sliort focus lenses, and under the inlluence of Lccuwen- hoek attention was returned to the development of the simple microscope. Antony von Lceuwen- hoek (1032-1723) constructed very efficient and convenient simple miscroscopes, developing the method already tried by llooke and Hartsoeker of making high-power lenses by allowing a drop of molten glass to occupy a small hole in a plate of brass. Even a drop of water or oil was also used in this way. Lecuwenhoek is said to have made 247 miscroscopes, observing the circula- tion of the blood in the feet of frogs, spermatozoa, and many other interesting things. To this pe- riod belong also the names of Wilson (1708-88), Hartsoeker ( l(i5-1724), Stephen Gray (?-1730), Jan van Musschenbroek (1087-1748), Leutmann (1067-1730), and others. About this time Samuel Reyher (1635-1714) employed such a lens to project an image upon the wall, or a screen, usin;.' the sunlight for illumination, and is thus probably the inventor of the 'solar microscope.' Baker ( l!i8-1774) with the aid of the mechanic Scarlett constructed in 1736 a catoptric miscroscope, using mirrors in- stead of lenses in a manner suggested by the Gregorian telescope. But such instruments never came to be of mucli importance, since Dolland (1700-01) in 1757 confirmed the theoretical con- elusions of Eulcr (1707-83) and Klingenstierna (l(!08-1705) that for the same refraction the dispersion might be dill'erent. and thereupon pro- ceeded to construct an achromatic objective, that is, a lens in which the color effects are elimi- nated by the use of two kinds of glass. Never- theless, the great difPicnlty of grinding such small lenses with sufficient accuracy for the correction of the errors due to aberration prevented their use in a manner at all commensurate with their successful employment in astronomical telescopes. In 1823 Sclligucs and Chevalier departed from the plan of using only two lenses to correct aber- ration and employed two or three pairs of lenses (see Fig. 6), each pair consisting of a plano-concave of flint j;Iass which dispersed the colors far apart, combined with a double convex of crown glass, which has a low dispersion. In this way excellent achromatic objectives were produced. In the next year TuUcy of London, ipon the suggestion of Or. Goring, constructed an nchromntie combination of three lenses, without knowing of (lie work of Selligiies and Chevalier. Amici of Modena had been endeavoring to pro- duce achromatic miscroscope objectives as early as 1812, and, encouraged by the success of Scl- ligues and Chevalier, he took up the work with new energi|-, and produced in 1827 a combination much superior to any known at that time. His work was soon rivaled by that of Andrew Ross and Powell in Lonilon. J. J. Lister, as a result of his theoretical investigations, directed James Smith in the construction of an objective that surpassed all others in the perfection of its cor- rection, singular aperture, and flatness of field. With these lenses A. Ross soon discovered that the presence or absence of a cover glass over the object affects the success of the correction. In other words, he discovered that the cover glass must' be considered as a part of the objective system. He pointed out that its effect may be counteracted by undercorrecting the first pair of lenses in the objective and overcorrccting the other two ])airs ; moreover, if the distance between the first and second pair of lenses of the objective can be varied, this makes it possible to adapt the correction of the objective to various thicknesses in the cover glass, and to various kinds of cover glasses. For a long time the best microscope objectives of high power were composed of three pairs of achromatic lenses, but Amici himself tried a single plano-convex lens next to the object and recently this has become quite popular. (See Fig. 7.) Amici also pointed out that where very short focus lenses are used a drop of water may be introduced between the cover glass and the first face of the objective, thereby reducing the loss of light. It is, however, evident that this would affect the refraction and dispersion of the system and hence throw out the correction. Ap- parently Amici was never able to adapt his sys- tems to this method of use, and it remained for Hartiiack and Nachet to succeed in constructing objectives for such use, and to point out their great superiority in many ways over the older form, which caiiie to be called 'dry' objectives, in distincti(m from this new form, which were called 'immersion' objectives. The immersion system has very great advantages over the dry on account of the gain in light by avoiding the strong reflection from the front lens in air, also because the correction of the cover glass is greatly simplified, and besides the range or work- ing distance is considerably increased. Naturally a lense constructed for immersion cannot be used satisfactorily for dry work, hut ^lessrs. Powell and Lealand so arranged their objectives that hv exchanging the front lens it could be changed from dry to immersion, or vice versa. Wenham still further improved upon this by so construct- ing the system that the objective could be changed from one form to the other by simply changing the distance between the first and sec- ond elements of the svsteni, this being accom- plished by turning a screw as in correcting for cover glasses in dry systems. See Fig. 0. Wenham also seems to have been the first to suggest the advantage of substituting for water a liquid which should have the same dis- persion and refraction as the cover glass and first lens of the objective, and it is to the zeal and energv of Zeis of .Tena. under the able guid- ance of llr. .bbe. that is due the almost perfect objectives which are available al the present da.T. The complex form shown in Fig. 7 is due to .Mibi'. and is known as an 'apochromat :' its cor-