2. The Objective (F)

The Objective (F). This, in the line of vision, is removed farthest from the eye and is the most essential portion of the instrument; to its many improvements (also further susceptibilities) are

Fig. 476.

I Description of the Microscope 1104

Ocular (Huyghenian).

Fig. 477.

I Description of the Microscope 1105

Ocular (Continental).

(and will be) due the great advancements in vegetable and animal histology. It consists usually of a front plano-convex lens (convexity upward), together with one, two, or three others, which may vary in shape between plano-convex, concavo-convex, and bi-convex (according to manufacture), working in combination as a single magnifier, thus

Fig. 478.

I Description of the Microscope 1106

Objective, 2/3-inch.

Fig. 479.

I Description of the Microscope 1107

Objective, 1/6-mcb.

Fig. 480.

I Description of the Microscope 1108

Oil-immersion objective, 1/12-inch.

producing an enlarged inverted and reversed image - this simple image being that which the ocular receives, and in turn magnifies. Objectives may be either dry or immersion, according as they are used without or with a drop of liquid between the lens and the object; if the liquid used has same refractive power as the glass lens, then it is called homogeneous immersion. Objectives are known by whole numbers and fractions, thus: 4 - 3- 2 - 1 1/2 - 1 - 3/4-inch = lower power; 1/4 - 1/5-inoh medium power; 1/6 - 1/8 - 1/10 - 1/12 - 1/16 - 1/25 - inch = high power, all of which are but expressions for intrinsic focal distance; a 1-inch objective, hence, has the same magnifying power as a simple lens with 1-inch focus, etc. The strength is inversely as these figures - smaller the fraction the greater the magnifying power, also smaller the end lens the greater the power; whereas working-distance, field of view, and amount of light are all directly as these figures - becoming less as the fraction diminishes. Objectives of low power, 2-1-inch, serve best for pharmacists, as these afford good working distance, hence the inspection of most objects without risk of injuring the lower lens.

The following table shows approximate magnifying powers of oculars and objectives taken conjointly under the indicated combinations:

Fro. 481.

I Description of the Microscope 1109

Objective nose-piece.

Oculars.

Objectives.

3 in.

2 in.

1 1/2 in.

l in.

2/3 in.

1/2 in.

Jin.

1/5 in.

1/6 in.

i in.

1/10 in.

1/12 in.

1/16 in.

A - 2

inch.

11

16

23

30

55

88

190

230

265

345

382

580

820

B - 1 1/2

,,

16

22

33

41

74

120

260

315

340

460

525

810

1100

C - l

,,

21

30

44

53

96

155

355

410

450

590

701

1060

1420

D - 1/4

,,

28

40

56

69

130

200

455

550

585

770

900

1400

1860

E - 1/2

,,

35

54

70

85

165

265

560

710

750

980

1110

1760

2350

This compilation is based upon tube length being 160 Mm. - 6 3/10 inches; for instruments whose tube length is 216 Mm. - 8 1/2 inches - an increase of about 25 p. c. must be added in each case.

Several higher objectives are made - 1/40 - 1/50 - 1/80-inch, which magnify 2,000-5,000 diameters according to ocular used. These all contain, in addition to the 2 - 4 lenses for magnifying, a combination of lenses for correcting chromatic and spherical aberrations, the most of which, however, is compensated for by the opposite aberration of the ocular.

Chromatic Aberration. - The lens proper is not only a magnifier, but also a prism; owing to this latter fact a ray of light in passing through it is deflected from its course and resolved into its elementary colors, thus giving, unless rectified, colored marginal bands around the image of the objects examined. This chromatic defect is overcome satisfactorily by a combination of lenses having opposite aberrations, namely, a convex crown lens and a concave flint lens, acting together as a single convex lens.

Spherical Aberration. - The convex lens, being in the nature of a prism, refracts rays of light toward the axis of the lens, but as the angle of the lens is greatest at the edge and gradually diminishes toward the centre, where the faces are parallel, the rays of light going through the edge of the lens are refracted more and come to a focus nearer the centre of the lens; hence there is a want of focus of the rays, or spherical aberration. This defect interferes with the defining power - i, e., the definition of the image - the image of a flat object becoming curved and blurred around its edges, so that in examining netted fabrics the central fibres are straight and distinct, the outer curved and indistinct. This spherical defect may be corrected by a diaphragm which cuts off border light by contracting the central aperture.