In 1823 Selligues and ('hevalie• departed from the plan of using only two lenses to correct aber ration and employed two or three pairs of („,, Fig. 6), each pair consisting of a of flint glass which dispersed the colors far apart. cmnhincd with a double convex of glass. which has a low ilispe•sion. Ito this way excellent achromatic objectives were produced. In the next year Tullcy of 1.011,1011, upon the suggestion of Dr. (boring. constructed an aehromatie combination of three lenses, without knowing of the work of Sellignes and Chevalier.
midi of Modena had been endeavoring to pro duce achromatic miseroseope objectives as early as 1812, and, encouraged by the success of Sel ligues and Chevalier, he took up the work with new energy, and produced in 1827 a combination much superior to any known at that time. Ilis work was soon rivaled by that of Andrew Ross and Powell in London. 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, angular 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 undereorreeting the first pair of lenses in the objective and overcorrecting the other two pairs: 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 Amiei himself tried it single piano-convex lens next to the object and recently this has become quite popular. (See Fig. 7.) Amiei 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 a ffeet the refraction and dispersion of the system and hence throw out the correction. Ap parently Amiei was never able to adapt his sys tems to this method of use, and it remained for Hartnaek 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 came to be called 'dry' objectives, in distinction from this new form. which were
called 'inmme•sionn' 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 tense constructed for immersion cannot be used satisfactorily for dry work. but Messrs. Powell and Lea laud so arranged their objectives that by exchanging the front lens it could be changed from dry to immersion, o• 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 ehanging the distance between the first and see ond elements of the system, this being necom by turning a screw as in eorrecting for cover glasses in dry systems. See Fig. (3.
Wenham also seems to have been Hie 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 energy of Zek of Jena. under the able gnid •unee of Dr. Abb.% that is doe the almost perfect objeetives which are available of the present day. The eomplex form shown in Fig. 7 is due to .\bh(, and is known as an `apochromat its eor rections are so perfect that it appears that the theoretically ideal conditions have been reached. Bausch and Lomb in America and Carl 'Geis in Germany are now constructing lenses under the specifications of Dr. Abl% It should be stated in this connection that the present great success in the construction of lenses of all sorts is in large measure due to the manufacture by Schott of Jena of glass upon scientific principles, so that it is not only possible to get glass with the same optical properties in large quantities, and at any time, but it has been possible to make glass with just those optical properties which are wanted for any particular purpose. Objec tives designed to be used as immersion lenses with a liquid of refraction and dispersion identi cal with that of the glass in contact with the liquid are called 'homogeneous' immersion lenses. Oil of cedar and oil of fennel are well adapted to use with such objectives.