xvi ILLUSTRATIONS. 



FIG. PAGE 



21. Absorption Spectrum of Iodine (Thalen) 42 



22. Method of Observing the Absorption of a great thickness of Vapour . . 42 



23. Keduced Copy of Becquerel's Photograph of the Complete Solar 



Spectrum taken in 1842 47 



24. Diagram showing the Solar Spectrum and the Bright Double Line D of 



Sodium 55 



25. Diagram illustrating the Graphical Formulae employed on p. 62 .... 63 



26. The Telluric Lines. Taken from the Atlas to Angstrom's Spectre 



Normal 67 



27. Copy of Janssen's Map of the Telluric Lines from C to D 67 



28. Spectroscope with Reflected Scale 7 



29. SteinheiPs form of Four-prism Spectroscope 71 



30. Steinheil's Slit, showing Reflecting Prism 72 



31. Path of Light through Comparison Prism 73 



32. Arrangement of Apparatus for observing the Spectra of Substances 



Volatilised by the Electric Spark 75 



33. Electric Lamp 84 



34. Angstrom's Spectrometer 85 



35. Sun-spot (Secchi) 95 



36. Total Eclipse (Dawes), 1851 97 



37. The Eyepiece End of the Newall Refractor (of twenty-five inches aperture) 



with Spectroscope attached . . . 99 



38. Spectrum of Sun-spot, showing the widening of the D Lines 100 



39. Arrangement for the Comparison of Spectra 103 



40. Mr. Hastings's Arrangement for Comparison of Spectra ...'.... 103 



41. Line C (red) with Radial Slit 108 



42. Line D 3 (yellow) with Radial Slit 109 



43. Line F (blue-green) with Radial Slit 109 



44. Spectrum of the Sun's Photosphere and Chromosphere, showing the Lines 



ordinarily visible, and that they extend to different heights . . . Ill 



45. Diaphragm showing Annulus, the breadth of which may be varied to 



suit the state of the air 114 



46. The Annulus is viewed and brought to focus by looking through apertures 



in the sides of the tubes . 114 



47. Solar Prominences (Young) showing Lateral Currents 115 



48. Edge of Chromosphere (billowy) 117 



49. Edge of Chromosphere (pointed) 117 



50. A Solar Prominence on the Disc (Young) 118 



