Page:The American Cyclopædia (1879) Volume XI.djvu/531

 MICROSCOPE 513 and Polynesia. Micronesia and Melanesia oc- cupy the western portion of the Pacific. The former extends from the westernmost island of the Hawaiian group to near Japan and the Philippines, and reaches S. of the equa- tor, comprehending the Marshall and Gilbert groups, the Ladrones, and the Carolines. Me- lanesia embraces the Feejee islands, the New Hebrides, Solomon's islands, New Caledonia, New Britain, New Ireland, and Papua. MICROSCOPE (Gr. fj.inp6<;, small, and <r/co7reZv, to see), an optical instrument for the examina- tion of minute objects. Microscopes are of two kinds, simple and compound. With the former, the object is viewed directly, either by means of a single lens or a set of lenses em- ployed in the same manner as a single lens. With the latter, an enlarged image of the ob- ject is formed by a single lens or a set of lenses, termed the object glass or objective; this image is viewed and further amplified by means of an eye piece or " ocular." Each form is valuable in its place, but as a general instrument of research the compound form, with all the modern improvements, is greatly superior. The invention of the simple micro- scope is not claimed by any one, but that of the compound has been warmly disputed ; it is claimed by the Italians and the Dutch. The compound microscope of the present day, however, is a very different instrument from the disputed invention, and to this last and best form we purpose to devote the most of our attention. The earliest magnifying lens known, if indeed it was used for this purpose, is the rude one found by Mr. Layard in the palace of Nimrud; it is made of rock crystal, and is far from perfect. Seneca (Quastiones Naturales, lib. i., cap. vi.) alludes to the magni- fying power of a glass globe filled with water ; he ascribes the effect to the water, and appears to refer to objects immersed in the water ; this was about 'the middle of the 1st century. " Burning spheres," as they are termed by Aris- tophanes, were sold in the shops of Athens in his day, about 400 B. C. There is no evidence that lenses were employed at this early date for magnifying, at least otherwise than as read- ing glasses. It is not until the 17th century that we find powerful magnifiers of glass actually employed for scientific investigation. The names of Malpighi, Lieberkiihn, Hooke, Leeu- wenhoek, Swammerdam, Lyonnet, and Ellis are closely connected with the history of the simple microscope; and their important dis- coveries attest the value of even this form of the instrument. Most of the magnifiers em- ployed by the early observers were minute sin- gle lenses of glass ; often small spheres formed by melting threads of glass in the flame of a spirit lamp. The small single lenses of high power are usually plano-convex, the plane side toward the object; when carefully made, hav- ing a focal length of from T V to 7 V of an inch, and well set in shallow blackened cells, with the proper aperture, they perform on ordinary objects tolerably well ; much better than the minute glass spheres, as the latter are difficult to obtain free from bubbles. The writer has succeeded better by melting a fragment of plate glass in a small hole in a German silver cup, by means of the blowpipe, and has formed lenses in this way, ready mounted for use, quite equal to those ground and polished by hand. The German silver is blackened by the heat. In order to diminish the spherical ab- erration in the high magnifier of a single re- fracting substance, Sir David Brewster suggest- ed the employment of gems ; and Mr. Pritch- ard of London, under the patronage of Dr. Goring, ground lenses of garnet, sapphire, and diamond, all of which proved superior to glass lenses of equivalent focus. The diamond lens- es upon the whole were inferior to the sap- phire ; the latter, though not as highly refrac- tive, were free from the veins which rendered several of the diamond lenses useless, though still affected, in common with the diamond, by double refraction. The garnet lenses are free from this latter defect, and when very minute are much superior to glass ; the color is not objectionable when the lenses are very small. All the magnifiers composed of single lenses, glass or gems, are surpassed by the doublets and triplets. The invention of the doublet in its best form is due to Dr. Wollaston ; it appears, however, to have been a chance discovery. It consists of two plano- convex lenses, having their plane sides to- ward the object ; the posterior lens (that near- est the eye) is three times the focal length of the anterior, and the distance between them is twice the focal length of the shorter. It is evident that the front lens of the doublet must be approached much nearer to the object than if it had been used alone, and the amplification is also less than that of the front lens alone; hence the working distance is much less than that of an equivalent single lens. The great and surpassing advantage of the doublet is the enlarged angle of aperture and diminution of spherical aberration. By "angle of aperture" is meant the angular breadth of the cone of rays proceeding from the object, and refracted through the lens or set of lenses. Evidently, with a single lens, having an aperture equal to its focal length, the angle will be about 55 ; in other words, the lines drawn in the same plane from a point to the margin of the lens, this point being in the axis of the lens, and at a distance from its convex surface equal to the diameter of the lens, will be 55; no single lens, however, will admit anything like this aperture. In the doublet the front lens is approximated much closer to the object than it possibly could be if employed alone, and hence it admits a wider angle ; the reduction of mag- nifying power, at the same time, diminish- es spherical aberration, which is still further reduced by the peculiar relations of the cur- vatures. The doublet thus becomes a very superior instrument, and, when well made and