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. 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. ALTITUDE. The height in degrees of the sun, or any heavenly body, above the horizon.