[Ill.u.s.tration: FIG. 389.--"Near sightedness", or myopia. Parallel rays come to a focus at _F_; emerging rays focus at _A_, the far point.]

=392. Defects of Vision.=--There are several defects of vision that may be corrected by spectacles or eye-gla.s.ses. One of these is "near-sightedness." It is due either to an eyeball that is elongated, or to an eye lens that is too convex, or to both conditions. This condition brings light from distant objects to a focus too soon (as shown in Fig.

389). Only light from near objects will focus upon the retina in such cases. With _normal_ vision light from _distant_ or _near_ objects may be focused without unusual effort upon the retina, see Fig. 390. The remedy for near-sightedness is to use concave lenses which will a.s.sist in properly refracting the light so the focus will be formed on the retina (Fig. 391). "Far-sightedness" is the reverse of near-sightedness; the eyeball is either too short, or the lens too flat, or both conditions obtain, so that the light entering the eye is brought to a focus behind the eyeball (Fig. 392). The remedy is convex lenses which will a.s.sist in properly converging the light, see Fig. 393. A third defect is called _astigmatism_. This is caused by some irregularity or lack of symmetry in the eye. It is corrected by a _cylindrical_ lens that compensates for this defect of the eye. A diagram similar to Fig.

394 is used as a test for astigmatism. If the lines appear with unequal distinctness, some irregularity of refraction (astigmatism) is indicated.

[Ill.u.s.tration: FIG. 390.--The normal eye. The parallel rays _A B_ focus without accommodative effort at _C_.]

[Ill.u.s.tration: FIG. 391.--Correction of near-sightedness by concave lens.]

[Ill.u.s.tration: FIG. 392.--Far-sightedness or hyperopia. Parallel rays focused behind the retina.]

[Ill.u.s.tration: FIG. 393.--Correction of far-sightedness by a convex lens.]

[Ill.u.s.tration: FIG. 394.--Test card for astigmatism.]

=393. The Photographic Camera.=--This is a light-tight box, provided with a convex lens in front, covering an aperture and a ground gla.s.s screen at the back. The distance between the lens and the screen is adjusted until a sharp image is obtained upon the latter, which is then replaced by a sensitive plate or film. The sensitized surface of the plate or film contains a salt of silver which is changed by the action of light. After the plate has been "exposed" to the action of light, it is "developed" by the use of chemicals producing a _negative_ image.

From "negative," by the use of sensitized paper, "positive" prints may be secured which resemble the object photographed.

[Ill.u.s.tration: FIG. 395.--Diagram of the projecting lantern.]

=394. The projecting lantern= (see Fig. 395) employs a strong source of light, as an electric arc lamp _L_, to strongly illuminate a transparent picture, or _lantern slide_, _S_, a real image (_I_) of which is formed upon a large screen. Two large plano-convex lenses (_C_), called condensing lenses, are placed near the lamp to concentrate the light upon the "slide" _S_. The convex lens forming the image is called the "objective" (_O_).

=395. The compound microscope= consists of two lenses. One called the _objective_ is placed near the object to be viewed. This lens has a short focal length usually less than a centimeter. It forms a _real image_ of the object. _A'_-_B'_. The other lens, the _eyepiece_ forms a virtual image of this real image. _A''_-_B''_. (See Fig. 396.)

=396. The telescope= consists of two lenses, the eyepiece and the objective. As in the compound microscope, the objective of the telescope forms a real image of the distant object, the eyepiece forming an enlarged virtual image of the real image. It is the virtual image that is viewed by the observer. (See Fig. 397.) In order to collect sufficient light from distant stars the objective is made large, sometimes 50 in. in diameter.

[Ill.u.s.tration: FIG. 396.--Formation of an image by a microscope. _A_-_B_ is the object. _B'_-_A'_ the real image formed by the "objective."

_B''_-_A''_ is the virtual image formed by the eyepiece. The eye sees the virtual image.]

The length of the telescope tube depends upon the focal length of the objective, since the distance between the two lenses must equal the _sum_ of their focal lengths.

[Ill.u.s.tration: FIG. 397.--Formation of an image by a telescope. _b_-_a_ is the real image; _d_-_c_ is the virtual image seen by the observer.]

=397. The opera gla.s.s= consists of a convex lens as objective and a _concave_ lens as an eyepiece. The former tends to form a real image but the latter diverges the rays before a real image can be formed, the action of the two lenses producing an enlarged virtual image (as in Fig.

398) which is viewed by the one using the gla.s.s. The compact size of the opera gla.s.s is due to the fact that the distance between the two lenses is the _difference_ of the focal lengths.

[Ill.u.s.tration: FIG. 398.--Formation of an image by an opera-gla.s.s.

_a_-_b_ is the virtual image.]

[Ill.u.s.tration: FIG. 399.--Diagram of the Zeiss binocular or prism field gla.s.s.]

=398. The Prism Field Gla.s.s or Binocular.=--This instrument. has come into use in recent years. It possesses the wide field of view of the spy gla.s.s but is as compact as the opera gla.s.s. This compact form is secured by causing the light to pa.s.s back and forth between two right-angle prisms (as shown in Fig. 399). This device permits the use of an objective lens with a focal length three times that of the tube, securing much greater magnifying power than the short instrument would otherwise possess. A further advantage is secured by the total reflection from the two prisms, one of which is placed so as to reverse the image right for left and the other inverts it, so that when viewed in the eyepiece it is in its proper position.

Important Topics

1. The eye: parts, formation of image, kind, how, where.

2. Eye defects, how remedied. Visual angle.

3. Simple microscope, camera; images, kind, how formed.

4. Compound microscope, telescope and opera gla.s.s; images, action of each lens.

Exercises

1. Name three instruments in which lenses form virtual images and three in which _real_ images are formed.

2. In what direction is an oar in water apparently bent? Explain by a diagram.

3. What optical instruments have you used? Is the _visible_ image formed by each of these _real_ or _virtual_?

4. The focal length of a copying camera lens is 14 in. Where must a drawing be placed so that an image of the same size may be formed upon the ground gla.s.s screen? What must be the distance of the screen from the lens?

5. What are two methods by which you can determine the focal lengths of the lens of a photographic camera?

6. The critical angle for water is 48-1/2 degrees. Show by a diagram how much of the sky can be seen by a diver who looks upward through the water.

7. How is near-sightedness caused? How is it corrected? Ill.u.s.trate by a diagram.

8. How is the eye accommodated (focused) as an object gradually approaches it?

9. Explain why a simple microscope a.s.sists in looking at the parts of a flower or insect.

10. Why do people who have good eyesight when young require gla.s.ses as they grow old?

(7) COLOR AND SPECTRA

[Ill.u.s.tration: GUGLIELMO MARCONI

"Copyright by Underwood & Underwood, N. Y."

Guglielmo Marconi (Italy). Inventor of wireless telegraphy.]

[Ill.u.s.tration: ALEXANDER GRAHAM BELL

"Copyright by Underwood & Underwood, N. Y."

Alexander Graham Bell, Washington, D. C. Inventor of the telephone.]

=399. Color.=--Much of the pleasure experienced in gazing at beautiful objects is due to the _color_ shown by them. The blue sky, the green gra.s.s, and the varied tints of flowers, and of the rainbow all excite our admiration The study of color begins naturally with the production of the _spectrum_, the many-colored image upon a screen produced by pa.s.sing a beam of light through a prism. The spectrum is best shown when the light enters by a narrow slit (Fig. 400). The spectrum was first produced by Sir Isaac Newton in 1675 by the means just described. The names usually given to the more prominent colors of the spectrum are violet, indigo, blue, green, yellow, orange, and red. The initials of these names, combined, spell _vibgyor_, a word without meaning except to a.s.sist in remembering the order of the colors in a spectrum. If the light that has pa.s.sed through a prism is sent through a second prism placed in reverse position (see Fig. 401), the light pa.s.sing through both prisms is found to be white. This experiment _indicates that white light is composed of light of all colors_.

[Ill.u.s.tration: FIG. 400.--Formation of the spectrum by a prism.]

[Ill.u.s.tration: FIG. 401.--The colors of the spectrum recombine to form white light.]

=400. Dispersion.=--The separation of the colors by a prism is called dispersion. In experimenting to find a reason for dispersion, it has been learned that lights of different colors are of different wave lengths. Color in light is therefore a.n.a.logous to pitch in sound. We hear through many octaves, but we see through about one octave. That is, the shortest visible waves of violet light are about 0.000038 cm. in length while the longest visible red rays are 0.000076 cm., or the longest visible light waves are about twice the length of the shortest visible ones. It appears from the evidence of experiments upon dispersion that _light waves of different lengths are refracted differently_. This causes the images formed by refraction through simple gla.s.s lenses to be fringed with color and to lose some of their sharpness and definiteness of outline, since the violet light is brought to a focus sooner than the red. (See Fig. 402.) This seriously affects the value of such lenses for optical purposes. Fortunately it is found that _different kinds of gla.s.s have a different rate of dispersion for the same amount of refraction_.