MICROSCOPE. 454: MICBOSCOPE. good results on account of the slight curvature of the face nearest the ohject ; y is the original liollaiid triplet in which the diaphragm cuts olf the stray light and improves the correction great- ly, a result attained in e and d hy the side cuts in toward the axis. Wollaslon pointed out that the improvement in using the two lenses is in the fact that the aberration of one is in large measure corrected by the other, the diaphragm serving to cut od' that portion of stray light which would interfere with the distinctness of the image. The field of vision is also larger and more nearly flat than when a single lens is used. CoMi'orNi) JIicuo.scoi'K. In its simplest form as invented by .lanssen the compound microscope consists of two lenses as shown in Fig. 4. The jb b' FlO. 4. 8IMIM.F. COM- ror.ND. FlO. 5. COMPOlTNn MICROSCOI'K WITH IIDVtiBNH EYEPIECE. so-called ohjcetive lens cd forms a greatly en- larged image of the object, aft, at a'h'. The eye- piece Im is a simple microscope, or magnifying glass, and the eye of the observer is at e. The magnifying power of such a eond)ination is ob- tained as follows: the image a'h' is larger than the object in the pnipnrtion of h'c to ca, and the eyepiece Im niagnilics the image oV/ in the proportion of its focal length to the distance of distinct vision, 25 cm. In a particular case: .suppose VII is 0.2 cm., ch' is 2<t cm., and the focal length of Im is 2 cm. Then the imago </'/<' will lie larger than the object in the propor- tion of 20 to 0.2. i.e. 100; and the eyepiece Im will magnify the image in the ratio of 2.'> cm. to 2 cm., i.e. 12.."), and the total apparent increase in size will be 100 X 12.5, or 1250 diameters. The Huygens ej'epiece, so called from its inventor. Is also called a negative eyepiece, because the two lenses are too far apart to make its use possible in the same manner as other forms. The action of this eyepiece is shown in Fig. 5, and also on the accom])anying plate. The objective would form an image at na if it were not that the lens ff of the eyepiece is introduced, and consequently the combined effect is to form the image really at hh : this is then viewed by the eye-lens ee. diapliragni is inter]iosed at 6i to cut olT stray light and improve the distinctness, ff is called tile field lens of the eyepiece, and ee is the eye- lens. The great advantage of this form of eye- piece lies in the fact that the chromatic and spherical aberration of the field lens, ff, is op- posite and about equal to that of the eye-lens, ee. Although this lens is very satisfactory for gen- eral microscopic work, it is practically little used where it is necessary to use a micrometer (q.v. ) in the eyepiece, or a cross-hair. Naturally the most important optical part of the microscope is the objective, as upon its per- fection depend the satisfactory results of the whole conddnation. In its simplest form it is only a plano-convex lens with its flat side toward the object. As usually seen it is as shown in FlO. 6. SECTION' OV ZEIS OBJECTIVE. . 7. *AeocHIlO,lAT' OF AIIHE. Fig. C, with two or more achromatic pairs; the Zeis objective there shown also illustrates how the cover-glass correction is accom]dished by varying the distance between the first two and the last two pairs of the objective, by means of a screw, K. Fig. 7 illustrates the lenses of one of Dr. Abbe's most perfect objectives, the 'apo- chronuit.' In general the eyepiece must not he astigmatic, i.e. it must be able to form a sharp image of a point. It must be orthoscopic, i.e. it must magnify all parts of the image equally. It must be achromatic, i.e. it must not show any colors not really present in the object. The above characteristics must also be pos- sessed by the objective, even more essentially and jierfectly than the cyepieci'. In addition it is necessary to undersland what is meant by other lieculiaritics of the objective. Indcr "aperture' is meant the angle between the limiting rays of tile elTeetive lieam in the formation of the image by the objective, for example, the angle end or ibd. Fig. 4. This is naturally affected by the index of refraction of the metlitim between the object and the objective, and would hence be dif- ferent with the same objective if it were used dry. as water immersion, or homogeneous im- mersion, and consequently it has been proposed
