CONTENTS. 



cornea. 5. Aqueous humour, pupil, and Iris. 6. Crystalline 

 humour and ciliary processes. 7. Choroid. 8. Ketina and 

 vitreous humour. 9. Axis of the eye and optic nerve. 10. 

 Numerical data. 11. Limits of the play of the eye. 12. 

 Achromatism of the eye. 13-16. How vision is caused. 17. 

 Conditions of perfect vision. 18. Distinctness of the image. 

 19. Parallel rays. 20, 21. Defects of vision and their remedies. 

 22-24. Power of adaptation. 25-29. Limits of this power. 

 30, 31. Causes of defective vision. 32. Magnitude of the image 

 on the retina. 33. Apparent magnitude defined. 34-37. Nature 

 of its variation. 38-40. Diminutiveness of the pictures on the 

 retina. 41. Sufficiency of illumination ..... 



49 



CHAP. II. 42. Power of accommodation of the eye. 43, 44. Apparent 

 brightness of luminous objects. 45-47. Intensity of brightness. 

 48-57. The image must continue a sufficient time upon the 

 retina to enable that membrane to produce a perception of ifc 

 Various illustrations of this. 58. Conditions which determine 

 apparent motion. 59. How affected by distance. 60. Example. 

 61. When imperceptible. 62. Motion of the firmament. 63. 

 Objects in rapid motion invisible. 64-66. Duration of the im- 

 pression on the retina. 67. Optical toys. 68, 69. Coincidence 

 of the optical and geometrical centres of the eye. 70. Ocular 

 spectra and accidental colours. 71. Why visible objects do not 

 appear inverted. 72. The seat of vision.' 73-75. The optic 

 nerve insensible to light .65 



CHAP. III. 76. Why objects-are not seen double. 77. Exceptional 

 cases. 78. The eye has no direct perception of distance or mag- 

 nitude. 79. How distances are estimated. 80. Appearance of 

 the sun and moon when rising or setting. 81. How magnitudes 

 are estimated. 82. Illusion produced in St. Peter's at Rome. 

 83. Magnitude inferred from distance. 84. Perception of angular 

 motion. 85. How real direction of motion may be inferred. 86. 

 Examples of the sun and moon. 87-90. Effect of the motion of 

 the observer Examples. 91. Angular distance defined. 92. 

 The eye has no direct perception of form How inferred. 93. 

 Visible area defined. 94. Figure inferred from lights and 

 shadows. 95. Power of distinguishing colours. 96,97. Absence 

 of this power in particular cases 81 



THE ATMOSPHERE. 



Experimental proofs of the weight of the atmosphere. 2. 

 bladder glass. 3. Pressure equal in all directions. 4. Pressure 

 of air in a room explained. 5. Magdeburg hemispheres. 6. 

 Suction with tube. 7. Pascal's experiment at Rouen. 8. Horror 

 of a vacuum. 9. Galileo and the pump-makers. 10. Torricelli's 

 celebrated experiment. 11. Pascal's experiment on the Puy-de- 

 ddme. 12. Actual pressure of atmosphere ascertained. 13. 

 Height of an atmosphere of uniform density. 14. Vastly greater 



The 





