XXX.— THE TELEPOLARISCOPE 441 XXXI.— CELESTIAL PHOTOGRAPHY.—THE WAYS AND MEANS 454 XXXII.— CELESTIAL PHOTOGRAPHY (CONTINUED): SOME RESULTS 463 XXXIII.— CELESTIAL PHOTOGRAPHY (CONTINUED): RECENT RESULTS 469 LIST OF ILLUSTRATIONS. FIG. PAGE 1. The heavens according to Ptolemy 3 2. The zodiac of Denderah 7 3. Illustration of Euclid’s statements 10 4. The plane of the ecliptic 13 5. The plane of the ecliptic, showing the inclination of the 14 earth’s axis 6. The first meridian circle 20 7. The first instrument graduated into 360° (west side) 21 8. Astrolabe (armillæ æquatoriæ of Tycho Brahe) similar to the 26 one contrived by Hipparchus 9. Ecliptic astrolabe (the armillæ zodiacales of Tycho Brahe), 28 similar to the one used by Hipparchus 10. Diagram illustrating the precession of the equinoxes 31 11. Revolution of the pole of the equator round the pole of the 32 ecliptic caused by the precession of the equinoxes 12. The vernal equinox among the constellations, B.C. 2170 34 13. Showing how the vernal equinox has now passed from Taurus and 34 Aries 14. Instrument for measuring altitudes 35 15. Portrait of Tycho Brahe (from original painting in the 39 possession of Dr. Crompton, of Manchester) 16. Tycho Brahe’s observatory on the island of Huen 43 17. Tycho Brahe’s system 46 18. The quadrans maximus reproduced from Tycho’s plate 48 19. Tycho’s sextant 50 20. View and section of a prism 56 21. Deviation of light in passing at various incidences through 57 prisms of various angles 22. Convergence of light by two prisms base to base 59 23. Formation of a lens from sections of prisms 60 24. Front view and section of a double convex lens 61 25. Double concave, plane concave, and concavo-convex lenses 61 26. Double convex, plane convex, and concavo-convex lenses 62 27. Convergence of rays by convex lens to principal focus 62 28. Conjugate foci of convex lens 63 29. Conjugate images 64 30. Diagram explaining Fig. 29 64 31. Dispersion of rays by a double concave lens 65