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78. Mode of showing the image of a microscope as an object

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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

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93. Combination of the colours of a spectrum to form white light 94. Complementary colours

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95. Combination of two homogeneous colours

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96. Refraction and internal reflexion in a rain-drop

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97. Refraction and double internal reflexion in a rain-drop 98. Mode of formation of the rainbow

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99. Refraction and internal reflexion in a drop of water

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100. Theory of the rainbow

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101. Combination of two similar prisms without deflection and with

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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,
but without dispersion (an achromatic prism)

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.

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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

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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

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126. Absorption and fluorescing spectrum of Naphthalin-red 127. Construction of the thermopile

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130. Heat-curves of the spectra thrown by flint glass and rock salt. 201 131. Action of the invisible thermotic rays

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132. Light, heat, and photographic action of the solar spectrum 133. Fresnel's mirror

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142. Diffraction or inflection image of a narrow slit 143. Diffraction apparatus.

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149. Comparison of the prismatic with the grating spectrum 150. Newton's colour glass

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160. Wave-surface of a negative uniaxial crystal
161. Huyghens' construction of double refraction
162. Two rhombohedra of Iceland spar
163. Polarised ray of light

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.

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172. Crossed Tourmaline plates

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173. Two Nicol's prisms employed as a polariser

174. Decomposition of vibrations

175. Dubosq's polarising apparatus

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180. Rotation of the planes of vibration in Quartz. 181. Circular movement of pendulum

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187. Soleil's Saccharimeter.

185. Tube for the reception of circularly polarising fluids

186. Double plate of right and left rotating Quartz

188. Compensator .

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Erratum.

Page 92, four lines from top, for FM, read F M'.

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

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