Part 73

Mrs. B. Not an unanswerable one, I hope. The image on the retina, it is true, is reversed, like that in the camera obscura; as the rays, from the different parts of the landscape, intersect each other on entering the pupil, in the same manner as they do, on entering the camera obscura. The scene, however, does not excite the idea of being inverted, because we always see an object in the direction of the rays which it sends to us.

Emily. I confess I do not understand that.

Mrs. B. It is, I think, a difficult point to explain clearly. A ray which comes from the upper part of an object, describes the image on the lower part of the retina; but, experience having taught us, that the direction of that ray is from above, we consider that part of the object it represents as uppermost. The rays proceeding from the lower part of an object, fall upon the upper part of the retina; but as we know their direction to be from below, we see that part of the object they describe as the lowest.

Caroline. When I want to see an object above me, I look up; when an object below me, I look down. Does not this prove that I see the objects themselves? for if I beheld only the image, there would be no necessity for looking up or down, according as the object was higher or lower, than myself.

Mrs. B. I beg your pardon. When you look up, to an elevated object, it is in order that the rays reflected from it, should fall upon the retina of your eyes; but the very circumstance of directing your eyes upwards, convinces you that the object is elevated, and teaches you to consider as uppermost, the image it forms on the retina, though it is, in fact, represented in the lowest part of it. When you look down upon an object, you draw your conclusion from a similar reasoning; it is thus that we see all objects in the direction of the rays which reach our eyes.

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?