But I have a further proof in favour of what I have advanced, which, I hope, will remove your remaining doubts: I shall, however, defer it till our next meeting, as the lesson has been sufficiently long to-day.

Questions

1. (Pg. 157) What is optics?

2. (Pg. 157) What is meant by a luminous body?

3. (Pg. 157) What is meant by a dark body, and what by an opaque body?

4. (Pg. 157) What are transparent bodies?

5. (Pg. 157) What is a medium?

6. (Pg. 158) How is light projected from luminous bodies, and how, from every point of such bodies, (fig. 1, plate 15?)

7. (Pg. 158) Why do not the rays of light from different points, stop each other's progress?

8. (Pg. 158) What is a ray, and what a pencil of rays? fig. 2, plate 15.

9. (Pg. 158) Do we know whether light is a substance, similar to bodies in general?

10. (Pg. 158) When a ray of light falls upon an opaque body, what is the result?

11. (Pg. 159) In what does shadow consist?

12. (Pg. 159) Why are they, in general, but partially dark?

13. (Pg. 159) Upon what does the intensity of a shadow depend?

14. (Pg. 159) How are shadows affected by the size of the luminous body, as represented in plate 15, fig. 3?

15. (Pg. 159) When is the shadow larger than the intercepting body?

16. (Pg. 160) What is explained by fig. 4, plate 15?

17. (Pg. 160) What will be the effect of several lights, as in fig. 5, plate 15?

18. (Pg. 160) Why will neither of these shadows be very dark?

19. (Pg. 160) What becomes of the light which falls upon an opaque body?

20. (Pg. 160) What is meant by reflection?

21. (Pg. 161) What is meant by the incident, and reflected rays?

22. (Pg. 161) What is the result, when the incident ray falls perpendicularly, and what, when it falls obliquely?

23. (Pg. 161) What two angles are always equal in this case?

24. (Pg. 161) To what law in mechanics, is this a.n.a.logous, as represented in fig. 4, plate 2?

25. (Pg. 161) What is represented by fig. 6, plate 15?

26. (Pg. 161) By what light are we enabled to see opaque, and by what, luminous bodies?

27. (Pg. 161) What enables us to see a ray of light in its pa.s.sage, through a darkened room?

28. (Pg. 162) By what reasoning would you prove that an object, such, for example, as a house, is seen by reflected light?

29. (Pg. 162) Why may one side of such object appear more bright than another side?

30. (Pg. 162) How is the fact exemplified by the sun, or moon, shining upon water?

31. (Pg. 162) Why is this best evinced by moonlight?

32. (Pg. 163) By what light do we see the moon, and why is it comparatively feeble?

33. (Pg. 163) What circ.u.mstance, renders objects seen by moonlight, still less vivid?

34. (Pg. 164) What is meant by the pupil of the eye?

35. (Pg. 164) What by the retina?

36. (Pg. 164) How do the rays of light operate on the eye in producing vision?

37. (Pg. 164) How may this be exemplified, in a darkened room?

38. (Pg. 164) What is meant by a _camera obscura_?

39. (Pg. 164) How is it explained in plate 16?

40. (Pg. 165) Why are the objects inverted and reversed?

41. (Pg. 165) What a.n.a.logy is there between the camera obscura, and the eye?

42. (Pg. 165) Is it the object, or its picture on the retina, which presents to the mind an idea of the object seen?

43. (Pg. 166) By what organs is sensation produced, and how must these organs be affected?

44. (Pg. 166) How will the idea of contact, apply to objects not touching the eye?

45. (Pg. 167) Why do not objects appear reversed to the eye, as in the camera obscura?

CONVERSATION XV.