CONTENTS. v 



PACE 



fallacy of the fable of "the Dog and the Shadow." 19. Objects 

 seen at the bottom of a transparent body. 20. Case of water 

 and glass. 21. Broken appearance of a rod immersed in water. 

 22. Cases in which rays cannot emerge from a transparent 

 body. 23. Experimental verification. 24. Reflection by a 

 rectangular Prism. 25. Images produced by lenses. 26. Six 

 kinds of lenses. 27. The axis of a lens. 28. Example of each 

 kind of lens. 29. Optical image produced by a convex lens. 

 30. Relative position of the object and image . . .81 



CHAP. II. 31. Experimental verification. 32. Variation of the 

 magnitude of the image. 33. Principal focus and focal length. 

 34. Variation of position, and magnitude of image. 35. When 

 images real, and when imaginary. 36. Images produced by 

 concave lenses. 37. Focal length varies with refracting power. 

 38. Refracting power depends on material of lens. 39. Sphe- 

 rical aberration. 40. Images produced by lenses not absolutely 

 clear and distinct. 41. Series of images. 42. Nebulous and 

 confused effect. 43. Spherical aberration greater near the 

 borders. 44. Increases with the curvature. 45. And with 

 the magnifying power. 46. Spherical distortion. 47. Curved 

 images. 48. How to diminish spherical aberration. 49. Lenses 

 made from diamonds and other precious stones. 50. Ineffectual 

 attempts at improvement by this means. 51. Methods of dimi- 

 nishing spherical aberration by proper adaptation of curvatures. 

 52. Aplanatic lenses. 53. Chromatic aberration. 54. White 

 light compound. 55. Coloured lights sometimes compound. 

 56. Images produced by homogeneous lights. 57. Images pro- 

 duced by compound light. 58. Lenses always produce several 

 images of a natural object. 59. Why they are not always so 

 confused as to be useless for vision. 60. Dispersion. 61. Dis- 

 persion increases with refraction. 62. Dispersion different with 

 different material . . . . . . .. .97 



CHAP. III. 63. Experimental illustration. 64. Dispersive powers. 

 65. Dispersive power does not necessarily increase with refrac- 

 tive power. 66. Example of the diamond. 67. Achromatic 

 lens. 68. Achromatic combination of flint and crown-glass. 

 69. Form of the compound lens . . . . .113 



COMMON THINGS THE LOOKING-GLASS. 



1. Though common and familiar, little understood. 2. Image of an 

 object produced by it vertically erect, but laterally inverted. 

 3. Image of an object parallel to the looking-glass. 4. Image of 

 an object inclined to the reflector. 5. Series of images formed 

 by two reflectors. 6. Example in rooms furnished with several 

 mirrors. 7. Reflection by a looking-glass takes place at the 

 posterior surface' silvering. 8. Analysis of the effect of a 

 looking-glass upon the light falling on it. 9. Conditions on 

 which the goodness of a looking-glass depends. 10. Effect of 

 mirrors flush with the floor. 11. Best method of cleaning 



