CONTENTS. vu 



Example of the bust of Cuvier. 5. Principle of the stereoscope. 



6. Origin of the name. 7. Wheatstone's reflecting stereoscope. 



8. Sir David Brewster's lenticular stereoscope. 9. Method of 

 obtaining stereoscopic pictures. 10. How the effects of relief are 

 produced. 11: Natural relief greatly exaggerated. . . 137 



COMETS. 



CHAP. I. I. COM ET ART ORBITS : 1. Prescience of the astronomer. 

 2. Strikingly illustrated by cometary discovery. 3. Motion of 

 comets explained by gravitation. 4. Conditions imposed on the 

 orbits of bodies which are'Subject to the attraction of gravitation. 

 5. Elliptic orbit. 6. Parabolic orbits. 7. Hyperbolic orbits. 

 8. Planets observe in their motions order not exacted by the 

 law of gravitation. ; 9. Comets observe no such order in their 

 motions. 10. They move in conic sections, with the sun for the 

 focus. 11. Difficulty of ascertaining in what species of conic 

 section a comet moves. 12. Hyperbolic and parabolic comets 

 not periodic. 13. Elliptic comets periodic like the planets. 

 14. Difficulties attending the analysis of cometary motions. 15. 

 Periodicity alone proves the elliptic character. 16. Periodicity 

 combined with the identity of the paths while visible establishes 

 identity. 17. Many comets recorded few observed. 18. Classi- 

 fication of the cometary orbits. II. ELLIPTIC COMETS REVOLVING 

 WITHIN THE ORBIT OF -'SATURN: 19. Encke's comet. 20. Table of 

 the elements of the orbit. 21. Indications of the effects of a 

 resisting medium. 22. The luminiferous ether would produce such 

 an effect. 23. Comets would ultimately fall into the sun . .145 



CHAP. II. 24. Why like effects are not manifested in the motion of 

 the planets. 25. Corrected estimate of the mass of Mercury. 26. 

 Biela's comet. 27. Possibility of the collision of Biela's comet with 

 the earth. 28. Resolution of Biela's comet into two. 29. Changes 

 of appearance attending the separation. 30. Faye's comet. 31. 

 Reappearance in 1850-1 calculated by M. Le Verrier. 32. De 

 Vice's comet. 33. Brorsen's comet. 34. D'Arrest's comet. 

 35. Elliptic comet of 1743. 36. Elliptic comet of 1766. 37. 

 Lexell's comet. 38. Analysis of Laplace applied to Lexell's comet. 

 39. Its orbit before 1767 and after 1770 calculated by his 

 formulae. 40. Revision of these researches by M. Le Verrier. 

 41. Process by which the identification of periodic comets may 

 be decided. 42. Application of this process by M. Le Verrier to 

 the comets of Faye, De Vico, and Brorsen, and that of Lexell 

 their diversity proved. 43. Probable identity of De Vice's 

 comet with the comet of 1678. 44. Blainplan's comet of 1819. 

 45. Pons's comet of 1819. 46. Pigott's comet of 1783. 47. 

 Peters's comet of 1846. 48. Tabular synopsis of the orbits of 

 the comets which revolve within Saturn's orbit. 49. Diagram of 

 the orbits. 50. Planetary character of their orbits. III. ELLIP- 

 TIC COMETS, WHOSE MEAN DISTANCES ARE NEARLY EQUAL TO THAT 

 OF URANUS : 51. Comets of long periods first recognised as periodic. 



