78. Mode of showing the image of a microscope as an object 104 86. 88. Action of Gregory's reflector 90. Different deflection of different coloured rays of light. 85. Action of Newton's reflecting telescope Action of the reflecting telescope with front opening 87. Gregory's reflecting telescope 89. Vaporisation of metal in the arc of the electric flame 91. Undecomposability of the colours of the spectrum 92. Impure spectrum obtained by the use of a circular opening 109 110 110 93. Combination of the colours of a spectrum to form white light 94. Complementary colours 95. Combination of two homogeneous colours 121 96. Refraction and internal reflexion in a rain-drop 123 97. Refraction and double internal reflexion in a rain-drop 98. Mode of formation of the rainbow 124 125 99. Refraction and internal reflexion in a drop of water 126 100. Theory of the rainbow 129 101. Combination of two similar prisms without deflection and with 105. 107. Dispersion of colour of a lens 102. Combination of a crown and flint-glass prism causing disper 103-4. Combinations of prisms which cause no deflection Combination of a crown and flint-glass prism, with deflection, 106. Spectrum thrown by crown glass and by flint glass 108. Achromatic lens 109. Measurement of refraction as practised by Fraunhofer 110. Spectrometer 111. Bunsen's spectroscope 112. Induction apparatus 113. Geissler's spectrum tube. 137 FIG. 114. Action of the comparison prism 115. Comparing prism at the slit of the spectroscope 116. Bunsen's apparatus for the absorption of Sodium light 117. Absorption of the Sodium flame 118. Telescope with four prisms 119. Absorption spectra of nitrous oxide and of the vapour of iodine 173 120. Absorption spectra 121. Absorption of the colouring-matter of litmus with different thicknesses of the layer 175 178 126. Absorption and fluorescing spectrum of Naphthalin-red 127. Construction of the thermopile 190 199 130. Heat-curves of the spectra thrown by flint glass and rock salt. 201 131. Action of the invisible thermotic rays 202 132. Light, heat, and photographic action of the solar spectrum 133. Fresnel's mirror 142. Diffraction or inflection image of a narrow slit 143. Diffraction apparatus. 258 260 149. Comparison of the prismatic with the grating spectrum 150. Newton's colour glass 271 273 160. Wave-surface of a negative uniaxial crystal 164. Nicol's prism. 165. Polarisation by reflexion 166. Two polarising mirrors 167. Biot's polarising apparatus 168. Nörremberg's polarising apparatus 169. Nörremberg's polarising apparatus, with glass laminæ 170. Tourmaline tongs 171. Parallel Tourmaline plates. 289 290 293 172. Crossed Tourmaline plates 315 173. Two Nicol's prisms employed as a polariser 174. Decomposition of vibrations 175. Dubosq's polarising apparatus 316 317 325 180. Rotation of the planes of vibration in Quartz. 181. Circular movement of pendulum 334 336 187. Soleil's Saccharimeter. 185. Tube for the reception of circularly polarising fluids 186. Double plate of right and left rotating Quartz 188. Compensator . 347 348 349 350 OPTIC S. CHAPTER I. SOURCES OF LIGHT. 1. NONE of our senses supplies us with such extensive and exact knowledge of the external world as that of sight. The eye penetrates into the unfathomable abysses of space, and receives intelligence from regions the most remote and inaccessible; it reveals to us the delicate cells of which living beings are composed, and perceives the animalcules that people the waters, whilst the manifold forms which it discloses to the mind are rivalled only by the exquisite beauty and charm of colour with which the physical world appears to be decorated. The visual organ, like every other special sense, possesses a peculiar form of sensibility, that of perceiving luminous rays, a faculty which admits of no more precise definition and explanation than the corresponding sensations of sound or heat, of taste or smell. The sensation of light can only be excited in our minds by a stimulus of one kind or another acting upon the retina, which is the delicate expansion of the optic nerve lining the posterior part of the eye-ball. The B |