When describing the common refracting telescope, (p. 228.) I have noticed that three eye-glasses, placed at double their focal distances from each other, formerly constituted the terrestrial eye-piece, as represented in fig. 47. But this construction, especially for achromatic instruments, has now become obsolete, and is never used, except in small pocket spy-glasses formed with a single object lens. In its place a four glassed eye-piece has been substituted, which is now universally used in all good telescopes, and which, besides improving the vision and producing an erect position of the images of objects, presents a considerably larger field of view. During the progressive stages of improvement made in the construction of erect eye-pieces by Dollond and Ramsden, three, four, and five lenses were successively introduced; and hence, in some of the old telescopes constructed by these artists, we frequently find five lenses of different descriptions composing the eye-piece. But four lenses, arranged in the manner I am now about to describe, have ultimately obtained the preference. In a telescope having a celestial eye-piece of the Huygenian form, the image that is formed in the focus of the object glass, is that which is seen magnified, and in an inverted position; but when a four glassed eye-piece is used, which produces an erect view of the object, the image is repeated, and the second image, which is formed by the inner pair of lenses AB on an enlarged scale, is that which the pair of lenses CD at the eye-end render visible on a scale still more enlarged. The modern terrestrial eye-piece, represented in fig. 79, is, in fact, nothing else than a compound microscope, consisting of an object lens, an amplifying lens, and an eye-piece composed of a pair of lenses on the principle of the Huygenian eye-piece. Its properties will be best understood by considering the first image of an object, which is formed in the focus of the object glass, as a small luminous object to be rendered visible, in a magnified state, by a compound microscope. The object to be magnified may be considered as placed near the point A, and the magnified image at i, which is viewed by the lens D. Hence, if we look through such an eye-piece at a small object placed very near the lens A, we shall find that it acts as a compound microscope of a moderate magnifying power increasing, in some cases, the diameter of the object about 10 times, and 100 times in surface.

figure 79.

In order to distinguish the different lenses in this eye-piece, we may call the lens A, which is next to the first image, the object-lens, the next to it B, the amplifying-lens, the third, or C, the field-lens, and the one next the eye, D, the eye-lens. The first image formed a little before A, may be denominated the radiant, or the object from which the rays proceed. Now, it is well known as a principle in optics, that if the radiant be brought nearer to the lens than its principal focus, the emerging rays will diverge, and, on the contrary, if the radiant be put farther from the lens than its principal focal distance, the emerging rays will converge to a point at a distance beyond the lens, which will depend on the distance of the radiant from the first face of the lens. In this place an image of the radiant will be formed by the concurrence of the converging rays, but in a contrary position; and the length of the image will exceed the length of the radiant in the same proportion, as the distance of the image from the radiant exceeds that of the radiant from the lens. This secondary image of the radiant at i, is not well-defined, when only one lens, as A, is used, owing to the great spherical aberrations, and therefore the amplifying lens is placed at the distance of the shorter conjugate focus, with an intervening diaphragm of a small diameter at the place of the principal focus; the uses of which lens and diaphragm are, first to cut off the coloured rays that are occasioned by the dispersive property of the object lens,—and secondly, to bring the rays to a shorter conjugate focus for the place of the image, than would have taken place with a single lens having only one refraction. As the secondary image is in this way much better defined and free from colouration, the addition of this second lens is a great improvement to vision. For this reason I am clearly of opinion, that the object glass of a compound microscope, instead of consisting of a small single lens, should be formed of two lenses on the principle now stated, which would unquestionably add to the distinctness of vision.

With respect to the proportions of the focal lengths of the lenses in this four glass eye-piece, Mr. Coddington states, that if the focal lengths, reckoning from A to D, fig. 79, be as the numbers 3, 4, 4 and 3, and the distances between them on the same scale, 4, 6, and 5, 2, the radii, reckoning from the outer surface of A, should be thus:—

A{First surface27 }nearly plano-convex.
{Second surface1 }
B{First surface9 }a miniscus.
{Second surface4 }
C{First surface1 }nearly plano-convex.
{Second surface21 }
D{First surface1 }double convex.
{Second surface24 }

Sir D. Brewster states, that a good achromatic eye-piece may be made of 4 lenses, if their focal lengths, reckoning from that next the object, be as the numbers 14, 21, 27, 32; their distances 23, 44, 40; their apertures 5.6; 3.4; 13.5; 2.6; and the aperture of the diaphragm placed in the interior focus of the fourth eye-glass, 7. Another proportion may be stated:—Suppose the lens next the object A, to be 1⅞ inch focal length, then B may be 2½ inches, C 2 inches, and D 1½; and their distances AB 2½; BC 3⅝; and CD 2⅜. In one of Ramsden’s small telescopes, whose object glass was 8½ inches in focal length, and its magnifying power 15.4, the focal lengths of the eye glasses were A 0.775 of an inch, B 1.025, C 1.01, D 0.79;—the distances AB 1.18, BC 1.83, and CD 1.105. In the excellent achromatic telescope of Dollond’s construction which belonged to the Duc de Chaulnes, the focal lengths of the eye glasses, beginning with that next the object, were 14¼ lines, 19, 22¾, 14; their distances 22.48 lines, 46.17, 21.45, and their thickness at the centre, 1.23 lines, 1.25, 1.47. The fourth lens was plano-convex, with the plane side to the eye, and the rest were double convex lenses. This telescope was in focal length 3 feet 5½ inches.

The magnifying power of this eye-piece, as usually made, differs only in a small degree from what would be produced by using the first or the fourth glass alone, in which case the magnifying power would be somewhat greater, but the vision less distinct, and were the lens next the eye used alone without the field glass, the field of view would be much contracted. Stops should be placed between the lenses A and B, near to B, and a larger one between C and D, to prevent any false light from passing through the lenses to the eye. The more stops that are introduced into a telescope—which should all be blackened—provided they do not hinder the pencils of light proceeding from the object, the better will the instrument perform.

For the information of amateur constructors of telescopes, I shall here state the dimensions of two or three four glassed eye-pieces in my possession, which perform with great distinctness, and present a pretty large field of view. In one of these, adapted to a 44½ inch achromatic, the lens A, next the object, is 1⅞ inch, focal length, and about 1 inch diameter, with the plane side, next the object. The focal length of the lens B 21/10 inches, diameter 7/10 inch, with its plane side next A; distance of these lenses from each other 24/10 inches. Distance of the field lens C from the lens B 5½ inches. The small hole or diaphragm between A and B is at the focus of A, and is about 1/6 inch diameter, and about 3/8 of an inch from the lens B. The field lens C is 2 inches focal length, and 1¼ inch diameter, with its plane side next the eye. The lens next the eye D is 1 inch focal distance, ½ inch diameter, and is distant from the field glass 1¾ inch, with its plane side next the eye. The magnifying power of this eye-piece is equivalent to that of a single lens whose focal length is half an inch, and with the 44½ inch object glass produces a power of about 90 times. The lens next the eye can be changed for another 1⅜ inch focal length, which produces a power of 65; and the two glasses CD can be changed for another set, of a longer focal distance which produces a power of 45 times. The whole length of this eye-piece is 11½ inches.

In another eye-piece, adapted to a pocket achromatic, whose object glass is 9 inches focal length, the lens A is 1 inch focal length, and ½ inch diameter; the lens B 1¼ inch, and ½ inch diameter, their distance 1½ inch, the lens C 11/10 inch focal length, and 5/8 inch diameter; the eye-lens D 5/8 inch focal length, and 3/8 inch diameter; distance between C and D 1⅛ inch. The distance between B and C 1¾ inch. The whole length of this eye-piece is 4½ inches, and its power is nearly equal to that of a single lens of ½ or 6/10 of an inch focal length, the magnifying power of the telescope being about 16 times. Another eye-piece of much larger dimensions, has the lens A of 2½ inches focal length, and ¾ inch diameter: the lens B 2¾ inches focus and 5/8 inch diameter; and their distance 2¾ inches; the lens C 2⅝ inches focus and 1⅛ inch diameter; the lens D 1¾ inch focus and ¾ inch diameter; distance from each other 2¾ inches. The distance between the lenses B and C is 4 inches. The magnifying power is equal to that of a single lens 1⅛ inch focal distance. When applied to an achromatic object glass 6 feet 7 inches focal length, it produces a power of about 70 times. This eye-piece has a moveable tube 9 inches in length in which the two lenses next the eye are contained, by pulling out which, and consequently increasing the distance between the lenses B and C, the magnifying power may be increased to 100, 120 or 140, according to the distance to which this moveable tube is drawn out. It has also a second and third set of lenses, corresponding to C and D of a shorter focal distance, which produce higher magnifying powers on a principle to be afterwards explained.