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27. Why cannot we see surrounding objects on entering a darkened room? 28. Why can cats and owls see well at night?

29. Why can an ostrich while facing front see directly behind itself? 30. What is the extent of our field of vision?

[of the page?

31. Why, when we read, do our eyes follow the printed or written lines 32. Why is it that with two eyes we do not see double?

33. Why does winking not interfere with vision?

34. If a bright object is suddenly withdrawn and replaced before the eye, why are we unable to detect that it had been removed?

35. If we hold a pencil several inches from our face, pointing at a small vase across the room, why when looking at the pencil is it distinct. and the vase indistinct and double?

36. What is the smallest square visible to the naked eye?

37. What optical instrument did the ancient Chaldeans possess?

38. What is a microscope?

39. Who invented the first compound microscope?

40. What is a Telescope? An Opera Glass?

41. What is a Stereoscope? A Camera Obscura?

42. What is polarized light and its uses?

43. What is the cause of twilight? 44. What is the cause of the rainbow?

45. Why are the colors in the secondary rainbow reversed?

46. What causes a lunar rainbow?

47. What is the cause of the Aurora Borealis ?

48. What effect has a prism on a ray of light?

49. Are all the rays of the Solar Spectrum, rays of light?

50. Which are the longest waves of light? The shortest?

51. How many vibrations per second must there be to produce red light, white light and violet light?

52. How many kinds of spectra are there?

53. Explain the practical uses of the spectroscope.

[spectroscope?

54. How small a fraction of incandescent metal can be revealed by the 55. What is Phosphorescence, or Eremacausis, and its cause? [tern," etc.? 56. What is an Ignis Fatuus, or "Will o' the Wisp," "Jack o' the Lan57. Of what is color the result? Is white or black a color?

58. Does color actually exist, or is it only in the mind's perception of it? 59. Describe the Electric Light?

66

60. Why does a blow on the head make one see stars"?

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1. An agent or force in nature which, acting on the organs of sight, renders objects visible.

2. The sun.

3. 1st, An organ or sense of sight; 2d, A luminous, incandescent or phosphorescent body; 3d, A luminiferous ether or transmitting medium. 4. 185,400 miles a second. Earlier and less accurate calculations give it as 195,000 miles a second. [absolutely.

5. Eight minutes and fourteen seconds. Practically it is, though not 6. About seven and a half times.

7. 64 times greater than the star's light.

8. A transparent body allows us to see objects through it. A translucent body transmits some light, but not enough to enable us to see objects through it. An opaque body does not transmit any light.

9. The Corpuscular or Emission Theory advocated by Descartes, but particularly elaborated by Newton, teaches that light is an emission of exceedingly minute, elastic particles from a luminous body, and that these, striking the eye, give rise to the various phenomena of vision. Huygens, a Dutch philosopher, is the originator of the now generally accepted Undulatory Theory; namely, that a luminous body in space, causes vibrations in the ether surrounding it, which are immediately transmitted in waves in every direction with astonishing rapidity, and produce upon your eyes the sensation of light. Ether is an elastic fluid filling all space and insinuating itself between the atoms and molecules of a body.

10. 1. Light is propagated equally in every direction from a luminous body; 2. It moves through a homogeneous medium in straight lines; 3. The intensity varies inversely as the square of the distance.

11. It is. The angle of incidence is equal to the angle of reflection. 12. Smooth, highly polished opaque surfaces. A mirror is a highly

polished opaque surface. There are three kinds of mirrors; viz., plane, concave and convex.

13. When rays of light fall on the surface of a plane mirror they are reflected, and we see the image in the direction of the reflected rays.

14. By varying the angle at which two mirrors are held, we may vary the number of images seen, and these increase in number as the angle decreases. The kaleidoscope, invented by Sir David Brewster, is constructed on this principle: pieces of colored glass are arranged in a tube so as to change position when the instrument is revolved; these are reflected in three mirrors at an angle of 50°.

15. The rays are reflected by the surface of the water and those rays which reach it first are reflected first; hence, all reflections are reversed.

16. Its light falls equally over all parts of the water, but only that portion which is at the right angle can reflect the light to our eyes.

17. A concave mirror is a highly polished, curved surface, like the inside of a hollow sphere, and produces an erect, virtual image larger than life. As we approach, it increases in size until we reach the focus, when it disappears; on receding, the image decreases in size, and finally becomes indistinct.

18. He set fire to the Roman fleet at Syracuse.

19. A convex mirror is the highly polished outside surface of a sphere; the image is smaller than life.

20. The diverting of oblique rays from their course when falling upon the surface of any transparent medium. 1st, The ray in passing from a denser into a rarer medium is bent from the perpendicular. 2d, In passing from a rarer into a denser medium, the ray is bent towards the perpendicular.

21. The bottom is seen in the direction of the refracted ray; this passing from water to air is refracted from the perpendicular, hence objects appear nearer the surface. This also explains why water seems more

shallow than it really is.

22. A mirage is an optical illusion caused by the refraction and reflection of rays of light when passing through strata of air of different densities.

23. Presbyopia is far-sightedness. It is remedied by using convex lenses which bring the focus forward on the retina.

24. Myopia is short-sightedness. It is remedied by concave glasses or lenses which throw the focus back on the retina.

25. About 12 inches.

26. A white object, in sunlight, can be seen at a distance of 17,250 times its diameter. A red object, 8,625 times its diameter, and a blue object, 5,500 times its diameter.

27. Owing to the contraction of the pupils by bright light; as they expand, rays are admitted and objects are discerned.

28. Their eyes are sensitive to small quantities of light. The pupils have great power of contraction and expansion.

29. Yes, because its eyes are placed laterally, making its field of vision very extensive.

30. It comprehends an arc of about 180°.

31. To keep the words in the line of vision.

32. When we direct both eyes to the same object, the lines of vision, or visual axes, meet at the object, and the two images formed on the retina coincide with, and cover each other, thus forming but one.

33. Because winking is practically instantaneous, while a certain definite time is required for an image to disappear.

34. Because of the persistence of visual impressions. The impression remains upon the retina just as in the case of winking above referred to.

35. The pencil being in the line of distinct vision is therefore distinctly perceived, while the vase though in the field of view, is not in the line of distinct vision, and hence it appears double and indistinct.

36. Ehrenberg has shown that it is about 1-405 of an inch.

37. The simple microscope.

38. An optical instrument for the examination of objects invisible to the naked eye. When simple it consists of one or more convex lenses for viewing the object directly; when compound it contains a magnifier or ocular through which the image is viewed.

39. The honor is claimed both by the Italian and the Dutch. History does not reveal the inventor's name.

40. A telescope is an optical instrument for viewing objects at vast distances. There are two kinds, reflecting and refracting. An Opera Glass consists of an object glass (double convex), and an ocular, or eye-piece (double concave). The image formed is erect and magnified. 41. A stereoscope is an optical instrument constructed to give an ap

pearance of reality to the object viewed; the image appears as if looked at separately by each eye. This principle of binocular vision is a very inportant one. A Camera Obscura is a combination of lenses adjusted to reflect upon a screen or table in a darkened room the image of all objects within its field.

42. Polarized Light has undergone double refraction through some such medium as Iceland spar. It vibrates only in one plane, whereas ordinary light vibrates in every conceivable plane. It enables the physicist to determine the molecular structure of bodies, and the chemist to analyze or test the quality of substances.

43. Higher regions of the atmosphere refract and reflect the rays of light before, or after, the sun is visible.

water.

44. The refraction and reflection of sunbeams by drops of falling In the primary rainbow, the ray enters the top of the drop, and after refraction and reflection, emerges at the bottom, being bent upward. 45. In the secondary rainbow, the ray enters at the bottom, and after refraction and reflection, emerges at the top, the colors being thus reversed. The secondary is a reflection of the primary bow.

46. A lunar rainbow is caused by the refraction and reflection of moonbeams by drops of falling water.

47. It is not definitely known; though from disturbances created in telegraph apparatus, it is supposed to be due to the electrical condition of the rarefied atmosphere of the upper regions. The current generated in the wires is frequently sufficient for the transmission of messages without using any battery.

48. It disperses it into a spectrum containing the seven primary colors: violet, indigo, blue, green, yellow, orange and red. The violet is the most refrangible, and the red the least.

49. No; there are also calorific, or heat rays, and actinic rays.

50. The longest and slowest rays of the spectrum are those of heat; those of light are shorter and faster; but those of chemical action are the shortest and quickest.

51. For red light, 482 Trillions; for white light, 500 Trillions; for violet light, 707 Trillions, per second.

52. Four: Continuous, banded, absorbent and abnormal.

53. The spectroscope has enabled the Scientist to read, in unerring lines, the physical and chemical constitution of the sun, and has recently

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