Online Encyclopedia

execution being assumed. Object details will only be well seen if the aberration circles are small in comparison. The size of these details in the image depends only on the magnification of the objective, M =A/fi', and can by appropriate choice of the focal length of the objective be brought to the right value. In the case of a suitable ocular magnification, the details will be well seen, while the aberration circles remain invisible. It is therefore possible to See also:

JUDGE (Lat. judex, Fr. juge)

judge the excel- lence of the focusing of objectives on the over-magnification, which they permit. DD=See also:

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

diaphragm. E. See also:

ABBE (Serbo-Croatian Rai))

Abbe, through the so-called delicate K1K2=See also:

CONDENSER

condenser. See also:

ray transmission, suggested a way by which L=front See also:

lens of the the quality of the images of objectives can be objective. observed.

The ray transmission, shown in The See also:

LOWER

lower figure fig. 55, is obtained by means of a stop shows the See also:

plan of of the See also:

FORM (Lat. forma)

form shown in the lower figure the transmission. and placed under the condenser in the See also:

PLANE

plane of the See also:

IRIS

iris diaphragm. The entrance 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 is in this way reduced on two small See also:

SEPARATE

separate See also:

FIELDS, JAMES THOMAS (1817-1881)

fields, which nevertheless contain rays of all zones. It is necessary that the outside edge of the diaphragmcoincides with the edge of the entrance pupil. This can be attained by See also:

DRAWING

drawing the iris diaphragm so far as to form the entrance pupil. The See also:

double diaphragm is then in such a position that the edge of the See also:

OUTER

outer diaphragm coincides with the edge of the iris diaphragm. The object employed must have distinct boundaries. Abbe's test See also:

PLATE

plate consists of an object See also:

carrier on which six See also:

COVER (from the Fr. convert, from couvrir, to cover, Lat. cooperire)

cover glasses of exactly determined thickness (between 0.09 mm. and 0.24 mm.) are cemented. The cover glasses are silvered on their under surfaces, and in the silvering See also:

FINE

fine lines are See also:

DRAWN

drawn; these lines form the test object. This plate admits at the same See also:

time of a correct determination of the thickness of the cover See also:

glass, for which the best correction exists. So See also:

long as the object is not sharply focused two separate See also:

dispersion figures will be seen. The defects of the objective are revealed, e.g. two adjacent See also:

sharp images are formed, which become indistinct if they coincide, or one See also:

PENCIL (Lat. penicillus, brush, literally little tail)

pencil produces a distinct, the other an indistinct image, or that the images are surrounded with coloured rings.

Owing to the curvature of the image, all parts of the object are not seen distinctly at one and the same time. The resolving power of an objective depends on its numerical See also:

APERTURE (from Lat. aperire, to open)

aperture. The numerical aperture can be determined in two ways. A diaphragm with a very narrow hole is placed on the See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage, and the microscope sharply focused on the edges of the hole. The See also:

ILLUMINATING

illuminating See also:

MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror is turned aside and a graduated See also:

scale is laid on the See also:

FOOT

foot of the microscope. Strong systems produce in the proximity of their back focal plane an image of the scale, which can be inspected with a weak See also:

AUXILIARY (from Lat. auxilium, help)

auxiliary microscope, and the length of the visible See also:

PART

part of the See also:

GRADUATION (see also GRADUATE)

graduation determined. The ratio of See also:

HALF

half the length of the visible piece of the scale to its distance from the diaphragm on the stage gives the tangent of half the angular aperture. The sine of this See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle is the numerical aperture for dry lenses. With weak systems no auxiliary microscope is necessary, the See also:

eye-piece being removed and the scale viewed directly in the tube. E. Abbe constructed a See also:

SIMPLE

simple See also:

INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument for the determination of the aperture, termed the apertometer (fig. 56).

A semi-circular A°moo° .n~/~b//1e; AA 0 d ~'- C.Zeiss Abbe's See also:

JENA

Jena Apertometer pi At. View from above See also:

Side view glass plate bears two scales, over which two See also:

black thin See also:

metal plates See also:

bent back at right angles may be moved. A little hole in the silvered plate a marks the centre of this circle. Through this hole the points of the metal plates b can be observed by See also:

TOTAL

total reflection on the See also:

SURFACE

surface c. The apertometer is laid on the stage, so that the hole lies in the See also:

AXIS (Lat. for " axle ")

axis of the microscope, and the hole is sharply focused. The eyepiece being removed the image of the metal plates b produced by the objective is seen. In See also:

order to ensure for the eye a central position, there is fixed on the upper end of the tube in See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place of the eyepiece a disk of pasteboard or metal with an axial hole. The metal plates b are then moved till the points just cut off the edge of the See also:

field to be surveyed. The angular or numerical aperture can then be read off. With strong systems the vanishing of the points is observed with an auxiliary microscope, formed by means of the inner tube. In See also:

IMMERSION (Lat. immersio, dipping),

immersion systems the immersion liquid is placed between the front lens and apertometer. If the numerical aperture be known the resolving power is easily found.

The resolving power can also be determined by using different fine test See also:

OBJECTS

objects. Norbert's test plates, which See also:

bear graduated See also:

groups of extremely fine and narrow divisions are very useful, while the tests of Amphipleura pellucida and Surirella gemma are often employed. The magnification of a microscope is determined from the focal lengths of the two See also:

OPTICAL

optical systems and the optical tube length, for N =250 0/fl'See also:

F2(X2+Y2)

f2. To determine the optical tube length 0, it is necessary to know the position of the focal planes of the objective and of the ocular. If one focuses an auxiliary microscope, carried in the inner tube, on the image situated in the back focal plane of the objective of a distant object, and then on the dust particles lying on a slide pressed against the end of the outer tube, the displacement of the auxiliary microscope gives the distance of the back focal plane of the objective from the end of the outer tube. To determine the position of the anterior focal plane of the eyepiece, the eyepiece is placed on the stage with the eye-lens downwards. An auxiliary microscope is now focused first on the image of a distant object and then on the plane of the edge of the setting. This plane can be marked by a small piece of See also:

paper. This gives the distance of the anterior focal plane of the eyepiece from the bottom edge of the setting of the eyepiece and consequently also of the edge of the eye-piece carried by the upper end of the tube. These measurements determine the optical tube length A. There are many methods for determining the focal length of the objective. The objective to be examined is placed on the stage, and in the manner just shown, the distance of the focal plane from the edge of the fittings or to the surface plane of the front lens is deter-See also:

MINED

mined.

Any plane object a few yards distant can be used. If the object can be seen by using the mirror, the plane mirror must be used; then the actual size of the object and of the image produced by the objective is measured (of the image by a See also:

MICROMETER (from Gr. wcp6c, small, jArpov, a measure)

micrometer ocular). The distance of the object from the nearer See also:

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

focus of the objective is next determined. This distance is composed of the distance of the object from the centre of the plane mirror, and of the distance of the focus of the objective on the stage plate from the centre of the plane mirror. Let the size of the object be y, the size of the image y' the distance of. the object from the focus x, then y/y' =x/fi from which fi can be calculated (see LENS). The same method can be used to determine the focal length of the eyepiece. These are the dimensions necessary for determining the magnification of the micro-See also:

SCOPE (through Ital. scopo, aim, purpose, intent, from Gr. o'KOaos, mark to shoot at, aim, o ic07reiv, to see, whence the termination in telescope, microscope, &c.)

scope, viz. the optical length of the tube A, the focal lengths of the objective fi', and of the eyepiece f2. The focal length of an objective can be more simply determined by placing do objective micrometer on the stage and reproducing on a See also:

SCREEN (usually, but very doubtfully, connected with Lat. scrinium, a box for holding books, from scribere, to write; a connexion with Ger. Schranke, barrier, has been suggested)

screen some yards away by the objective which is to be examined. If the size of the image of a known See also:

INTERVAL

interval of the objective micro-See also:

METER, ELECTRIC

meter is determined by an See also:

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

ordinary scale, and the distance of the image from the focal plane of the objective belonging to it is measured, then the focal length can be calculated from the ratio y/y' =fi'/xi', in which y is the size of the object, y' that of the image, and x1' the distance of the image from the focal plane belonging to it. Besides this indirect method of determining the magnification there is also a See also:

DIRECT

direct one, in which it is not necessary to first measure f2 or A. If a drawing See also:

PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism is used above the eyepiece, and an objective micrometer is inserted, then if a scale is laid on the drawing See also:

BOARD (O. Eng. bord)

board which is 25 cm. distant from the exit pupil, one or more intervals of the objective micrometer can be seen projected on the scale lying on the board. The comparison of the two scales gives directly the magnification.

The course of the See also:

LIGHT

light within the drawing prism must be taken into See also:

account when determining the distance of the scale from the exit pupil. Although this method does not give very accurate results, it is more convenient and simple than the indirect method.

End of Article: THE TESTING OF THE

Additional information and Comments

There are no comments yet for this article.

» Add information or comments to this article.

Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.

Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
THE SYMPATHETIC

[next]
THE THYMUS