183. Spectroscope arranged for showing absorption 409 184. Geissler’s tube 413 185. Spectrum of sun-spot 415 186. Diagram explaining long and short lines 416 187. Comparison of the absorption spectrum of the sun with the 418 radiation spectra of iron and calcium, with common impurities 188. Comparison prism 423 189. Comparison prism 423 190. Foucault’s heliostat 424 191. Object-glass prism 426 192. The eyepiece end of the Newall refractor 427 193. Solar telespectroscope (Browning’s form) 428 194. Solar telespectroscope (Grubb’s form) 428 195. Side view of spectroscope 429 196. Plan of spectroscope 429 197. Cambridge star spectroscope elevation 430 198. Cambridge spectroscope plan 430 199. Direct-vision star spectroscope (Secchi) 431 200. Types of stellar spectra 433 201. Part of solar spectrum near F 436 202. Distortions of F line on sun 438 203. Displacement of F line on edge of sun 439 204. Diagram showing the path of the ordinary and extraordinary 445 ray in crystals of Iceland spar 205. Appearance of the spots of light on the screen shown in the 446 preceding figure, allowing the ordinary ray to pass and rotating the second crystal 206. Appearance of spots of light on screen on rotating the second 447 crystal, when the extraordinary ray is allowed to pass through the first screen 207. Instrument for showing polarization by reflection 448 208. Section of plate-holder 456 209. Enlarging camera 458 210. Instantaneous shutter 460 211. Photoheliograph as erected in a temporary observatory for 461 photographing the transit of Venus in 1874 212. Copy of photograph taken during the eclipse of 1869 474 213. Part of Beer and Mädler’s map of the moon 476 214. The same region copied from a photograph by De La Rue 477 215. Comparison between Kirchhoff’s map and Rutherfurd’s 480 photograph 216. Arrangement for photographically determining the coincidence 481 of solar and metallic lines