The advantage of the reflecting telescope is, that mirrors whose focus is six feet, will magnify as much as lenses of a hundred feet: an instrument of this kind may, therefore, possess a high magnifying power, and yet be so short, as to be readily managed.

_Caroline._ But I thought it was the eye-gla.s.s only which magnified the image; and that the other lens, served to bring a diminished image nearer to the eye.

_Mrs. B._ The image is diminished in comparison with the object, it is true; but it is magnified, if you compare it to the dimensions of which it would appear without the intervention of any optical instrument; and this magnifying power is greater in reflecting, than in refracting telescopes.

We must now bring our observations to a conclusion, for I have communicated to you the whole of my very limited stock of knowledge of Natural Philosophy. If it enable you to make further progress in that science, my wishes will be satisfied; but remember, in order that the study of nature may be productive of happiness, it must lead to an entire confidence in the wisdom and goodness of its bounteous Author.

Questions

1. (Pg. 195) What is the form of the body of the eye? fig. 1, plate 21.

2. (Pg. 195) What is its external coat called?

3. (Pg. 195) What is the transparent part of this coat denominated?

4. (Pg. 195) What is the second coat named?

5. (Pg. 195) What opening is there in this?

6. (Pg. 195) What is the coloured part which surrounds the pupil?

7. (Pg. 195) The pupils dilate and contract, what purpose does this answer?

8. (Pg. 196) How could you observe the dilatation and contraction of the pupils?

9. (Pg. 196) What purpose is the choroid said to answer?

10. (Pg. 196) In what animals is the change in the iris greatest?

11. (Pg. 196) What are the three humours denominated, and how are they situated?

12. (Pg. 197) What is the part represented at _i i_, and of what does it consist?

13. (Pg. 197) What are the respective uses of the humours, and of the retina?

14. (Pg. 197) Why is it necessary the rays should be refracted?

15. (Pg. 197) How is this ill.u.s.trated by fig. 3 and 4, plate 21?

16. (Pg. 198) What causes a person to be short-sighted? fig. 5, plate 21.

17. (Pg. 198) Why does placing an object near the eye, enable such, to see distinctly? fig. 6.

18. (Pg. 199) A concave lens remedies this defect; how? fig. 1, plate 22.

19. (Pg. 199) What is the remedy, when a person is long-sighted? fig. 2.

20. (Pg. 199) Why does holding an object far from the eye, help such persons? fig. 3.

21. (Pg. 200) How is the eye said to adapt itself to distant, and to near objects?

22. (Pg. 200) Why are objects rendered indistinct, when placed very near to the eye? fig. 4, plate 22.

23. (Pg. 200) What is the single microscope, fig. 5, and how does it magnify objects?

24. (Pg. 201) How may objects be magnified without the aid of a lens?

25. (Pg. 201) Why can an object, very near to the eye, be distinctly seen, when viewed through a small hole?

26. (Pg. 201) Describe the double microscope, as represented in fig. 6, plate 22.

27. (Pg. 202) How does the solar microscope, (fig. 1 plate 23.) operate?

28. (Pg. 202) Why may minute objects be greatly magnified by this instrument?

29. (Pg. 202) In its more perfect form it has other appendages, as seen in fig. 2, what are they? and what their uses?

30. (Pg. 203) What is added when opaque objects are to be viewed? fig.

3.

31. (Pg. 203) In what does the magic lanthorn differ from the solar microscope?

32. (Pg. 203) What are the use and structure of the telescope, as shown in fig. 4?

33. (Pg. 204) When terrestrial objects are to be viewed, why are two additional lenses employed?

34. (Pg. 204) What part of the telescope performs the part of a microscope?

35. (Pg. 204) In what does the reflecting, differ from the refracting telescope?

36. (Pg. 204) What advantages, do reflecting, possess over refracting telescopes?

GLOSSARY.

ACCELERATED MOTION. Motion is said to be accelerated, when the velocity is continually increasing.

ACCIDENTAL PROPERTIES. Those properties of bodies which are liable to change, as colour, form, &c.

ACUTE.--See ANGLE.

AIR. An elastic fluid. The atmosphere which surrounds the earth, is generally understood by this term, but there are many kinds of air. The term is synonymous with _Gas_.

AIR PUMP. An instrument by which vessels may be exhausted of air.