3. Theory of Stationary Sensory Circles 4. Theory of Moveable Sensory Circles 5. The number of unexcited Sensitive Fields necessary for producing a Double Sensation 6. Sensory Illusion 7. Horizontal Section of the Eye 8. The Formation of an Image by a Convex Lens PAGE 17 18 30 32 33 9. Camera Obscura 10. Reversal of the Retinal Image . 55 57 11. Cause of Indistinct Retinal Images 12. Alteration in the Curvature of the Lens during Adjustment 13. Mode of observing Reflected Images in the Eye . 14. Reflected Images observed in the Eye 63 65 68 69 20. Production of a Shadow by the Blood-vessels of the Eye . 82 25. Mode of proving the existence of the Blind Spot . 92 26. The connection between the Nerve Fibres and the Rods and Cones 95 a FIG. 27. Formation of a Spectrum 28. Colours of the Spectrum 29. Colour Top 30. Method for Mixing Colours 31. Illustration of the action of the Spectral Colours upon the Retina 64. Connection of Tympanic Membrane with Hammer-bone 65. Chain of Ear-bones 66. Mode of Vibration of Ear-bones. 113 118 77. Change of angle in the arch of the Fibres of Corti 78. Gradual increase in width of the arch of the Fibres of Corti . 79. Monochord 80. Nodal Points 81. Siren 82. Savart's experiment for finding the Lowest Perceptible Tone 83. Savart's experiment for finding the Highest Perceptible Tone 84. Resonator. 250 85. Production of a Pure Tone by means of Tuning-fork and Resonator. 252 86. Analysis of Vibration into Fundamental Tone and 1st Harmonic 253 87. Complicated Form of Sound-wave 259 88. Displacement of the Phases of the Harmonic 264 89. Electro-magnetic Tuning-fork . 266 90. Representation of the degree of Roughness between each Interval of the Octave 91. Olfactory Cells. 2282 279 287 THE FIVE SENSES OF INTRODUCTION. IN the animal kingdom a number of organs have been developed which possess the property of making each organism acquainted with occurrences in the outer world, and which are therefore called the 'sensory organs.' They are found in their highest perfection in man, whose mental power surpasses in the same degree that of the rest of organised beings. Every sensory organ may be shown to be anatomically connected with the nervous system by means of nerve-trunks and nerve-fibres. Touch, sight, hearing, smell, and taste are inconceivable without the presence of a nervous system, even if the sensory organs were in their present full development. An eye, of which the optic nerve has been destroyed, can present to us no picture of the outer world; an ear, of which the auditory nerve has been severed, conducts no sound to us; an arm, of which the nerve is injured, can feel nothing. Such an B |