EYEPIECE (Pr., Oculaire ; Ger., Okular) The lens, or combination of lenses, which receives the image from the microscope objec tive and conveys it to the eye. The best known types of eyepieces are the Huyghenian, the Ramsden, and the Kellner.
The Huyghenian, which is the most popular, is a negative combination composed of two plano-convex lenses separated by the distance of half the sum of their foci. The lower lens of an eyepiece, that nearest the objective, is known as the field lens, and the other is the eye lens. A stop is placed between the field and eye lens at the principal focus of the latter.
The Ramsden, which is a positive and achro matic eyepiece, is a combination of two plano convex lenses with their convex surfaces inwards. This form is especially useful for micrometric purposes. The Kellner eyepiece is now rarely used ; it gives a very large field, but the defini tion is not equal to that of the Huyghenian. The eye lens is a combination of a biconvex (field lens) and a biconcave lens (eye lens).
The projection eyepiece, as its name implies, is designed exclusively for lantern and photo graphic work. The field lens of a projection eyepiece consists of a plano-convex lens, the eye lens being a biconvex combination of three lenses. The field of this eyepiece is very limited, but it gives fine definition ; the magnification is small, from 2 to 6 diameters. The compensating
eyepiece is designed to be worked in conjunction with apochromatic objectives. This eyepiece derives its name from being over-corrected to compensate for the under-correction of the apochromats. The compensating eyepiece works well with high-power, but indifferently with low power, achromatic lenses. The eye lens of the compensating ocular is plano-convex and the field-lens a biconvex triplet, the eye lens and field-lens being placed in close juxtaposition.
There are no universal standards for either diameter or magnification of eyepieces. The Royal Microscopic Society has adopted four standard sizes for eyepieces—namely, No. I. 0•9173 in. (2343mm.) ; No. II. •o4 in. (26.416 min.) ; No. III. 1427 in. (32•358 mm.) ; No. IV. 1441 in. (35•814 mm.). The variety of methods of classifying the initial magnification or power of an eyepiece is even more unsatisfactory than the lack of uniformity in size. In most English eyepieces the initial magnification is indicated by letters A—E, A representing the lowest power listed by the particular firm, giving a magnifica tion of 4 or 6 diameters ; the initial magnification of E would be from 12 to 20. On the Continent figures, not letters, are used to classify the power of the ocular, but they give little clue to the magnification.