Part 90

Mrs. B. Very well, Emily; I congratulate you on having invented a telescope. In figure 4, the lens C D, forms an image E F, of the object A B; and the lens X Y, serves the purpose of magnifying that image; and this is all that is required in a common refracting telescope.

Emily. But in fig. 4, the image is not inverted on the retina, as objects usually are: it should therefore appear to us inverted; and that is not the case in the telescopes I have looked through.

Mrs. B. When it is necessary to represent the image erect, two other lenses are required; by which means a second image is formed, the reverse of the first, and consequently upright. These additional glasses are used to view terrestrial objects; for no inconvenience arises from seeing the celestial bodies inverted.

Emily. The difference between a microscope and a telescope, seems to be this:--a microscope produces a magnified image, because the object is nearest the lens; and a telescope produces a diminished image, because the object is furthest from the lens.

Mrs. B. Your observation applies only to the lens C D, or object-glass, which serves to bring an image of the object nearer the eye; for the lens X Y, or eye-glass, is, in fact, a microscope, as its purpose is to magnify the image.

When a very great magnifying power is required, telescopes are constructed with concave mirrors, instead of lenses. These are called reflecting telescopes, because the image is reflected by metallic mirrors. Concave mirrors, you know, produce by reflection, an effect similar to that of convex lenses, by refraction. In reflecting telescopes, therefore, mirrors are used in order to bring the image nearer the eye; and a lens, or eye-glass, the same as in the refracting telescope, to magnify the image.

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-glass 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 illustrated 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.