Jltcnoscol'r•.. In its simplest form as invented by Janssen the compound microscope consists of two lenses as shown in Fig. 4. The Huygens eyepiece, 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 accompanying plate. The objective would form an image at no if it were not that the lens if of the eyepiece is introduced, and consequently the combined effect is to form the image really at LL; this is then viewed by the eye-lens ce. A diaphragm is interposed at Lb to cut off stray light and improve the distinctness. ff is called the field lens of the eyepiece, and cc 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, cc. 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 combination. 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 so-called objeotive lens cd forms a greatly en larged image of the object, oh, at The piece ha is a simple microscope. or magnifying glass, and the eye of the observer is at e. The magnifying Tamer of such a combination is ob tained as follows: the image o'h' is larger than the object in the proportion of to CU, and the eyepiece im magnifies the image (ro' in the proportion of its focal length to the distance of distinct %kiwi, em. In a particular ease: suppose co is 0.2 em.. et,' is 20 eat., and the focal length of lam is 2 em. Then the image o'h' will be l,•trger than the object in the propor tion of 20 to 0.2. i.e. 100; and the eyepiece int will magnify the image in the ratio of 25 cm. to 2 i.e. 12.5, and the total apparent increase in size will be 100 X 12.5, or 1250 diameters. The Fig. 6, with two or more achromatic pairs; the Zeis objective there shown also illustrates how the eover-glass correction is nct•om1r1i.hed 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. most perfect objectives. the 'apo In general the eyepiece must not be astigmatic, i.e. it must be able to form a sharp image of a point. It must be orthoseopie, i.e. it must magnify all parts of the image
equally. It must be aehromatie. i.e. it must not show any colors not really present in the object.
above characteristics must also be pos sessed by the objective, even inure essentially and perfectly than the eyepiece. in addition it is necessary to %dint is meant by other peeuliarities of the objective. [rider is meant the angle between the limiting rays of the 1V(' 111'8111 in the formation of the image by the ihjccticc. for example, the angle cad or ch,/, 4. This is naturally affected by the index of refraction of the medium let veers 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 to use the product of the sine of half this angle by the index of refraction, as indicating the ef fective aperture irrespective of the method of using the objective, and this constant is called the numerical aperture. The resolving power of an objective must not be confused with the mag nifying power, for theoretically any desired de gree of magnification can be obtained, but there is a definite limit to the resolving power set by diffraction phenomena, as pointed out by Dr. Abla".. Owing to the fact that a lens on account of diffraction is not able to form an actual point, as the image of a point, it is evident that if the little rings which are formed overlap, then no degree of further magnification can separate them, and they will confuse the vision. It has been shown that the success of au objective in gather ing in all the components due to diffraction is directly dependent upon the numerical aperture. Ablai has calculated that the theoretical limit of resolving power for an aperture of 180° would be lines about 120,000 to the inch, falling to about 95,000 for 107°. This has been nearly reached in some of the best instruments. The oretically two lines must be distant from each other at least X /2a, in order to be seen dis tinctly, where a is the numerical aperture and X is the wave length of the light.
In order to make use of the highest efficiency of the objective it is necessary to devote much attention to the concentration of the light upon the object in order that the image may be well lighted and also that the full aperture of the objective may be utilized. A form of condenser which is placed under the object is shown in the accompanying plate (Fig. 4) ; Sp is the mirror for reflecting the light into the condenser S, and the rest is mechanism for suitable adjustments. The adjoining figures show the section of such a condensing lens.