concave lens. The exception to the similarity of the results produced by lenses and reflectors is, that with the latter there is no chromatic dispersion, and the only sources of error are the aberration and spherical confusion, which are common to both spherical reflectors and lenses. For astronomical observations, however, in which case the rays incident upon the object-speculum are parallel, these sources of error are removed by making this speculum of a parabolic form, and another speculum, if it be used, of the form of the vertex of a prolate spheroid. There is great difficulty in procuring flint glass in pieces of large size without flaws, and we are consequently limited as to the size of the lenses of good quality that can be formed with such glass; and, without its use, we have not hitherto been able to form available achromatic object-glasses. Recourse is, therefore, had to parabolic or spherical specula in the formation of telescopes of large power for examining the heavens *. These specula are formed of metal, and the chief objection to them is the impossibility of producing an accurate surface. Even supposing its general form to be correct, there are always minute inequalities arising from the nature of the substance, which cause a waste or dispersion of light. Great pains are, consequently, taken in their construction to obtain the form and surface of the best possible quality †. MICROSCOPES. The microscope is an instrument for magnifying minute, but accessible objects. A convex lens is a microscope, but the imperfections of such an instrument have been already explained (p. 71), and the greater the power of the lens the * Sir William Herschel's largest telescope was 40 feet long, and the mirror 4 feet wide. Lord Rosse's largest telescope is 56 feet long, and the mirror 6 feet wide. The following description of the methods employed in forming and polishing parabolic reflectors is extracted in an abridged form from an account of Skerryvore Lighthouse, by Alan Stevenson, LL.B. F.R.S.E. M.I.C.E., Engineer of the Northern Light Board. "The reflector plate is formed of virgin silver and the purest copper, from the ingot, in the proportion of 6 oz. silver to 16 oz. of copper. The two metals are formed into pieces of the form of rectangular parallelopipeds about 3 inches in length, and the same in breadth, and are then tied together with wire, placed in the furnace, and united with a flux of burnt borax and nitre, mixed to the consistence of cream. The metal thus united is repeatedly passed through the rolling mill, and annealed in the furnace after each time of passing through, until it comes out a plate 28 inches square. It is then cut into a circular disc ready for hammering; and great care must greater will be these imperfections. For small magnifying powers, then, convex lenses may be used, as they are for spec be taken to keep the metal clean during the processes of hammering and polishing now to be performed. The beating is commenced on the edge and continued round and round till the center is gradually reached. After the disc has been raised sufficiently by this means, t is taken to a machine called the horse, and beaten with a wooden mallet upon the copper side, its concave face being turned about upon a bright steel head a (fig. 2), until it has nearly reached the proper height for the reflector, which is ascertained by a mould m (fig. 3). "After each course of raising with the wooden mallet the reflector is annealed, as follows: first damped with clean water, and dusted over with powder, composed of one pint of powdered charcoal to one ounce of saltpetre; then put on a clear charcoal fire, till the powder flies off and shows when it is duly heated. It is next plunged into a pickle, composed of one quart of vitriol in five or six gallons of water; and, lastly, washed with clean water and scoured with Calais sand. "The next step is to put the reflector into an iron stool, and, having drilled a small hole in its vertex, to describe a circle from this point with beam compasses, and cut the paraboloid to the proposed size. "The reflector is now hard-hammered with a planishing hammer, or planished, as it is called, on the bright steel head a; and then smoothed with a lighter hammer muffled with parchment. Then comes the finishing, called also, filling up to the mould, which is thus performed. It is constantly tried with the mould m, and those portions which do not meet it are marked with fine slate pencil, and then gone over with the muffled hammer, till every point touches the mould. Great care must be taken in this vrocess that no part of the surface be raised above the gauge, or the reflector would have to be re-formed with the wooden mallet, and the whole process repeated. The reflector is then tried with a lamp brought to its focus, and, if the whole surface is luminous, it is fit for polishing; but, if not, it must be again tested by the mould, and carefully filled up with the muffled hammer, till the result of the lamp test is perfectly satisfactory. "The edge of the reflector is next turned over to stiffen it, and the bizzle w (fig. 1), and back belt g (fig. 2), having been soldered on, the final process of polishing may be proceeded with. This process is commenced by scouring, first with a piece of pure charcoal of hard wood, and next with a mixture of Florence oil and finely-washed rotten stone, applied by means of a large ball of superfine cloth. The reflector is then cleansed with a fine flannel dipped in Florence oil, and afterwards dusted over with powder of well-washed whiting, and wiped out with a soft cotton cloth. Lastly, it is carefully rubbed by the naked hand with finely-washed rouge and tacles; but for obtaining good images with high magnifying powers a combination of lenses must be used. Small glass spheres are used as microscopes of high powers; but a thin lens composed of any more highly refractive substance is preferable; because, the focal length of the sphere measured from its center being but three semi-radii, the distance of the object from the surface is only one semi-radius, which prevents its being used in the examination of delicate objects. The refracting sphere is much improved as a micro scope by cutting a groove round it in a diametrical plane, and filling it up with some black opaque substance. By this contrivance the aperture is diminished, without contracting the field of view, and all the pencils are necessarily centrical. Microscopes have been made of diamond and sapphire, and the aberration is much less than with glass. Dr. Brewster clean water, and wiped with a smooth chamois skin. In all the polishing and cleansing processes some skill is required in the manipulation, as the hand must be moved in successive circles parallel to the lips of the reflector, and having their centers on the axis of the generating curve." The speculum of Lord Rosse's great telescope is composed of 1264 parts of copper and 589 of tin, fused together and cast in a mould, the bottom of which is formed of hoop iron bound closely together with the edges uppermost. By this means the heat is conducted away through the bottom so as to cool the metal towards the top, while the interstices between the hoops, though small enough to prevent the metal from running out, are sufficiently open to allow the air to escape. After casting, the speculum is annealed in a brick oven, which is heated almost to a red heat, and shut up with the speculum in it, and allowed to cool gradually. The speculum is then placed with its face upwards upon a turning apparatus, and the grinding and polishing performed entirely by the aid of mechanical contrivances, so that the proper parabolic form is accurately given to it. To test the work, the dial-plate of a watch is placed upon the top of a mast at 90 feet distance from the speculum, and the image of this dial-plate formed by the speculum, being viewed through an eye-glass properly placed the distinctness of this image denotes the accuracy of the speculum. employed, as a microscope, a drop of Canada balsam or turpentine varnish upon a thin plate of glass, of which the surfaces were exactly parallel. This is a very ready way of forming a plano-convex lens, and if kept free from dust will last some time. The compound or achromatic microscope consists of four lenses and a diaphragm, placed in the following order: the object-lens; the diaphragm; the amplifying lens, so called because it amplifies or enlarges the field of view; the field-lens; and the eye-lens. The relations between the focal lengths and intervals of the lenses, and the distance of the diaphragm from the object-lens, are determined, so that the combination may be achromatic, aplanatic, and free from spherical confusion. The field-lens and eye-lens form what is called the eye-piece; and the object-lens and amplifying lens form, or tend to form, an enlarged image of the object, in the focus of the eye-piece, which image is viewed through the eye piece. When the focus of the eye-piece is beyond the fieldlens, so that the image is formed between the amplifying lens and the field-lens, the eye-piece is called a positive eye-piece; but when the focus of the eye-piece is between the two lenses of which it is composed, in which case its effect corresponds with that of a concave lens, it is called a negative eye-piece. With a negative eye-piece the pencils proceeding from the amplifying lens are intercepted by the field-lens before forming an image, and the image is formed between the field-lens and the eye-lens, in the focus of the latter. The best microscopes are constructed with compound objectlenses, which are both achromatic and aplanatic; and by this means the aperture, and consequently the quantity of light, is much increased. Good compound lenses possessing the required properties have been formed of a concave lens of flint glass, placed between two convex lenses, one of crown glass, and the other of Dutch plate. The magnifying power of any refracting microscope or telescope may be practically found, by pointing the object-end of the instruments towards the light, and receiving the image of the object-glass formed by the other lenses upon a screen placed at the eye-end of the instrument, and at a proper distance from it, which may be determined by trial. Then the ratio of the diameter of the object-glass, or of the diaphragm, in the case of the compound microscope, to the diameter of its image upon the screen, gives the magnifying power of the telescope or microscope In all microscopes it is necessary to illuminate the object strongly, in consequence both of the diffusion of the small portion of light, received from the object, over the magnified image, and of the absorption of the light by the several lenses. The Reflecting Microscope.-In this instrument a concave speculum of short focal length is substituted for the objectlens. The object is placed on one side of the axis of the instrument, so that its perpendicular distance from the axis, together with the distance from the speculum of the point where this perpendicular meets the axis, may be a little greater than the focal length of the speculum. A small plane reflector is placed upon the axis of the instrument at the point where the perpendicular from the object meets it. This reflector is set at an angle of 45° to the axis, and having its plane perpendicular to the plane through the object and the axis. The object being strongly illuminated, the pencils of rays proceeding from it, after reflection at the plane reflector and concave speculum, tend to form a magnified image, but are intercepted by the field-glass of the negative achromatic eye-piece, called the Huyghenian eye-piece (p. 82); and the image formed after the transmission of the rays through the field-glass is viewed through the eye-glass. In the examination of small objects with a high power, it is necessary that the microscope should be perfectly free from all tremor, the slightest motion being so magnified as to prevent a good view from being obtained. Regard must be had, therefore, to solidity and accuracy in the fitting of all the joints and screws: in the choice of an instrument, and for a first-rate instrument, recourse should be had only to a maker of well-known talent, as many so-called opticians are mere sellers of articles of the qualities of which they are totally ignorant. The adjustment of the eye-piece should be obtained through the medium of a clamp and slow motion screw of the best kind *, in which the screw acts upon a spiral spring, and by means of which the adjustment for a good focus may be obtained with the greatest possible accuracy, and without the slightest tremor If the workmanship and fittings of the instrument appear to be satisfactory, a few test objects should be examined with it, to try the quality of the combination of lenses. Two common test objects are the scales of Podura plumbea, or Skiptail, a small wingless insect, the size of a flea, found in damp cellars, and the Navicula pleuroigma, * The best clamp, referred to in the text, is Dollond's, or a modification of Dollond's clamp. |