2. Normal oscillations under the action of forces varying as the distance 3. Energy of a particle in periodic motion 4. Composition of periodic motions 5. Combination of any number of oscillations 6. Combinations of oscillations at right angles to each other 7. Composition of linear vibrations of slightly different periodic times 15. Waves spreading from a disturbed region of finite size. 16. The principle of superposition . 17. Huygens' secondary waves 18. Refraction and reflexion of waves 19. Wave-front and wave surface ART. CHAPTER III. PRELIMINARY DISCUSSION OF THE NATURE OF LIGHT PAGE 35 37 37 38 41 43 45 47 50 29. The interference of light . 54 30. Calculation of the combined effects due to two separate sources 31. Conditions necessary for the experimental illustration of interference 55 32. Young's experiment. 33. Fresnel's experiments 34. Subjective method of observing interference bands 35. Observations with white light 36. Difficulty of illustrating simple interference phenomena by experiment 63 50. Shadows of a straight edge in parallel light 51. Shadows of a straight edge in divergent light 52. Shadow of a narrow lamina 53. Passage of plane waves through a slit 56. 57. 58. Zone plates Historical . PAGE 84 88 92 93 94 96 98 99 102 103 104 105 105 83. The formation of images without reflexion and refraction. Pin-hole photography CHAPTER VIII. THE PROPAGATION OF LIGHT IN CRYSTALLINE MEDIA. PAGE 140 142 144 144 146 148 150 84. The ellipsoid of plane wave propagation. Ellipsoid of elasticity 85. The optic axes 152 155 92. The axes of single ray velocity and of single wave velocity 165 98. Refraction through a crystal of Iceland spar 99. Nicol's prism 171 172 100. Double image prisms 173 101. Principal refractive indices in biaxal crystals 102. Conical refraction 105. Intensity of illumination in transmitted light 106. Observations of colour effects with parallel light 180 182 107. Observations with light incident at different angles 183 108. Uniaxal plate cut perpendicularly to the axis 109. Relation between wave velocities 184 185 110. Relation between ray velocities 187 ᎪᎡᎢ . 111. The surface of equal phase difference or isochromatic surface 112. Application of isochromatic surface to the study of polarization effects 113. Isochromatic curves in uniaxal crystals PAGE 188 190 191 114. Isochromatic curves in biaxal crystals 193 115. Biaxal crystals cut at right angles to the bisector of the angle 193 125. Small strains in a small volume may always be treated as 130. Shearing stress produced by combined tension and pressure at right angles 132. Equations of motion in a disturbed medium 134. Maxwell's theory 135. Differential equation for propagation of electric and magnetic 210 210 211 214 215 137. Direction of electric and magnetic forces at right angles to |