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THE EYEPIECE OR

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Originally appearing in Volume V18, Page 402 of the 1911 Encyclopedia Britannica.

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THE EYEPIECE OR OCULAR The eyepiece is considerably simpler in its construction than the See also:

OBJECTIVE, or OBJECT GLASS

objective. Its purpose in a See also:

MICROSCOPE (Gr. µucp6s, small, asenreiv, to %view)

microscope is by means of narrow cones of rays to represent at infinity the real magnified See also:

IMAGE (Lat. imago, perhaps from the same root as imitari, copy, imitate)

image which the objective produces. As, however, the See also:

OBJECT

object represents a real image, the problem is to project a transparent diapositive. It is therefore impossible to observe this image through an See also:

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary See also:

lens. Since many of the rays coming from the exit-See also:

PUPIL (Lat. pupillus, orphan, minor, dim. of pupus, boy, allied to puer, from root pm- or peu-, to beget, cf. "pupa," Lat. for " doll," the name given to the stage intervening between the larval and imaginal stages in certain insects)

pupil of the objective would not reach the See also:

eye of the observer at all, it is necessary, in See also:

order to make use of all of them, to See also:

DIRECT

direct the diverging rays forming the real image so that the whole of the See also:

LIGHT

light enters the eye of the observer. This is effected by a collective lens; it may be compared with the second See also:

PART

part of the See also:

CONDENSER

condenser See also:

SYSTEM

system of a projecting See also:

LANTERN (an adaptation of the Fr. lanterne from Lat. lanterna or laterna, supposed to be from Gr. Aa urritp, a torch or lamp, Aaµaew, to shine, cf. " lamp "; the 16th- and 17th-century form " lanthorn " is due to a mistaken derivation from " horn," as a m

lantern. The two most customary eyepieces consist in two simp:e piano-See also:

CONVEX

convex lenses, whose distance one from the other is equal to See also:

HALF

half the sum of the two See also:

FOCAL

focal lengths. One of these is the See also:

RAMSDEN, JESSE (1735-1800)

Ramsden eyepiece (fig. 37). If the real image produced by the objective coincides with the collective lens, only the inclination of the See also:

PRINCIPAL

principal rays is altered, the See also:

FORM (Lat. forma)

form of the See also:

CONE (Gr. Kwvos)

cone being affected only to a very small extent. The lens nearer the eye, which has about the same focal length as the collective lens, is distant from it by about its focallength. The eye-lens converts diverging pencils into See also:

PARALLELS

parallels.

Both lenses together form the exit-pupil of the objective behind the eye, lens, so that this image, the exit-pupil of the See also:

TOTAL

total system or the Ramsden circle, is accessible to the eye of the observer. It is possible to see the whole See also:

field through this pupil by slightly moving the See also:

head and eye. In practice the real image is formed not directly . r Lz L2 =collective-, L3 = eye-lens. DD =See also:

DIAPHRAGM (Gr. Scat/pay,aa, a partition)

diaphragm of the field of view. P"P"= Ramsden's circle, or exit-pupil of whole microscope. on the collective lens but a little in front of it, because otherwise all the particles of dust on the collective would also be seen magnified. In the other type, the Huygenian eyepiece (fig. 38), which is much more widely used, the collective lens is in front of the real image ; it alters the direction of the principal rays and somewhat diminishes the real image. In this type the eye-lens is about twice as powerful as the collective lens, and makes the rays parallel. Here also the exit-pupil is accessible to the eye and through it the whole field can be seen by moving the head and eye. In both eyepieces micro-meters or See also:

CROSS

cross-wires are used for measuring in the See also:

PLANE

plane of the real L2 = collective-, L3 = eye-lens. DD =diaphragm of the field of view.

P"P" = Ramsden's circle, or exit-pupil of whole microscope. image. The Ramsden eyepiece is the most convenient for this because this plane lies in front of the collective lens, and the objective image has not yet been influenced by the eyepiece. As both eyepieces are used with very small apertures (about f : 20) no See also:

ATTEMPT (Lat. adtemptare, attentare, to try)

attempt has been made to overcome the spherical aberrations, which are usually very slight; neither, as a See also:

RULE

rule, are the eyepieces chromatically corrected, care has only to be taken by a suitable choice of the distance of one lens from the other, that the coloured images derived from a colourless object should have the same apparent See also:

SIZE

size. Since, however, the difference of See also:

CHROMATIC (Gr. xpcoµaruc6s, coloured, from xpwµa, colour)

chromatic magnification cannot be overcome in powerful objectives, this See also:

ERROR (Lat. error, from errare, to wander, to err)

error is still further increased by the eyepiece. The difference of chromatic magnification cannot even be over- come in apochromats, and to See also:

CANCEL (from the Lat. cancelli, a plural diminutive of cancer, a grating or lattice, from which are also derived " chancel " and " chancellor ")

cancel this See also:

ABERRATION (Lat. ab, from or away, errare,, to wander)

aberration See also:

ABBE (Serbo-Croatian Rai))

Abbe devised the compensating ocular (fig. 39). The weak See also:

COMPENSATION (from Lat. compensare, to weigh one thing against another)

compensation oculars resemble a Huygenian eyepiece with achromatic eye-lens, whilst the more powerful ones are of a different construction. These eye-pieces are intentionally provided with a different chromatic magnification, which however is in opposition to that originating in the objective. They have also a shorter FIG. 39.-Compensating Eye-See also:

FOCUS (Latin for " hearth " or " fireplace ")

focus for red, and a longer one for pieces (Zeiss). See also:

BLUE (common in different forms to most European languages)

blue, and thus magnify the red image more than the blue; and as the objective gives a large blue and a small red image, the two cancel one another and a colourless image is produced.

These eyepieces are very convenient in use, for when they are changed the See also:

LOWER

lower focus always falls in about the same plane. In See also:

German and See also:

French microscopes the See also:

OPTICAL

optical length of the See also:

TUBE (Lat. tuba)

tube, when apochromats and compensation-eyepieces are used, is 18o mm. By multiplying the magnification of the objective by the number L3 on the eyepiece the total magnification of the microscope is obtained. By the magnification of the objective is meant the ratio of the distance of distinct See also:

VISION (from Lat. videre, to see), or SIGHT

vision to the focal length of the objective. As powerful achromatic objectives show See also:

DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences of chromatic magnification in the same way as apochromats, compensation eyepieces can be used in See also:

COMBINATION (Lat. combinare, to combine)

combination with these objectives.

End of Article: THE EYEPIECE OR

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