MICROSCOPE. 453 MICROSCOPE. rections are so perfect that it appears that the theoretically ideal conditions have been reached. Uausch anil Lunib in America and Carl Zeis in Germany are now constructing lenses under the specilica'tions of Dr. Ahljo. It should be stated iu this connection that the present great success in the construction of lenses of all sorts is in huge 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. Simple Microscope. A simple lens, or a combination of two or more lenses nearer to- gether than the sum of their focal lengths, and acting as a single lens, so used as to supplement the optical system of the eye and increase the apparent size of an object, is properly called a simple niiscroscope, or magnifying glass. The observer judges of the size of an object by the visual angle which it subtends. For example, AB. Fig. 1, appears larger than CD because the visual ( F I ...-- ....... f
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D B Fio. 1. angle AEB is greater than the visual angle CED. Any device which increases the visual angle which an object subtends makes it appear larger. It is impracticable to bring the object indefinitely near to the eye and thus enlarge the visual angle, because the accommodation of the normal eye does not enable it so to adjust its optical system as to see distinctly an object much less than 20-2.5 cm. (eight or ten inches) distant. In other words, the normal eye can bring to a sharp focus on the retina only such rays of light as are parallel or slightly divergent. If a convex lens is placed close in front of the eye and an uV B' ^^^Ac^--,, I L '^"^^^4^ ^^^^ ■^^^^^ ^ y L ■ object in front of it, and distant a little less than its focal length, as shown in Fig. 2, the lens will form a virtual image. I. of the object, O. at A'l?'. and the liglit issuing from LL is of such divergence as to be readilv brought to a focus upon the retina by the lenses of the eye, and hence vision is distinct, and the visual angle and apparent size of the object are increased. From a considera- tion of Fig. 1 it is evident that practi- cally the apparent increase in size is approxi- mately proportional to the decrease in distance between the object and the eye. Under the normal conditions that the distance IE, Fig. 1, is at least 20-25 cm. (8 to 10 inches), and we can see distinctly only a comparatively small area at once, the angle AEB is small and approxi- mately proportional to the ratio of AB to IE. hence' AEB ^ CED = FE -4- IE, and the visual angle is inversely proportional to the distance from the eye to the object. Applying this to Fig. 2 gives A'B' -r- AB = IC -f- OC, inasmuch as C is very close to the eye. IC is the distance of distinct vision, and 00 is practically the focal length of the lens C. It is hence evident that the magnifying power of a simple lens is equal to the ratio of its focal length to the distance of distinct vision. For example, a lens of a focal length of 1 cm. (tw'O-fifths of an inch) would magnify 25 cm. -V- 1 em. or 25 diameters. Magnifying powers are always given in 'diameters.' that is, in the magnifying of any linear dimension and not of the area of the object. Simple lenses of very short focus are not well adapted to obtaining very high magnifying power, on account of their chromatic and spherical aber- rations, which render the image so colored and indistinct that accurate work is impracticable. A form of stand for simple microscope especially convenient for biological work is shown on the Fig. 3. lenses for simple micboscope. accompanying plate (Fig. I). A is the lens, or lens combination; B is the table for holding the object, and D is the mirror for concentrating light upon the latter; C is the rack and ])inion enabling a convenient adjustment of the focus. Such instruments are useful for dissecting small organisms, and can be furnished with magnifying power up to 100 diameters. Fig. 3 shows several methods of obtaining strong combinations with less aberration, and without the cost of elaborate correction. Fraunhofor designed the doublet, a: b is a form used by Wilson; c is a so-called aplanatic triplet by Steinheil ; e is the original Coddington, modified to the form d by Brew'ster, and f is the conunon cylindrical lens that obtains
