A { First surface 27 }nearly plano-convex. { Second surface 1 }
B { First surface 9 }a miniscus. { Second surface 4 }
C { First surface 1 }nearly plano-convex. { Second surface 21 }
D { First surface 1 }double convex. { Second surface 24 }
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 1-7/8 inch focal length, then B may be 2-1/2 inches, C 2 inches, and D 1-1/2; and their distances AB 2-1/2; BC 3-5/8; and CD 2-3/8. In one of Ramsden’s small telescopes, whose object glass was 8-1/2 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 14-1/4 lines, 19, 22-3/4, 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 5-1/2 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.