INDEX OF FIGURES FIG. PAGE 1. Illustration of Mental Measurement.....xxxiii 2. Appunn’s Lamella ......... I 3. Galton’s Whistle . . . . . . . . .12 4. Edelmann’s Galton Whistle . . . . . . 13 5. Scripture’s Touch Weights ....... 15 6. von Frey’s Hair Æsthesiometer . . . . . . .16 ■7. Application of Hair to the Cutaneous Surface (von Frey) . . 16 8. von Frey’s Limen Gauge . . . . . . . 17 9. Regulating Apparatus for Limen Gauge . . . . 18, 19 10. Lehmann’s Acoumeter ........ 24 11. Correlation of Intensive Pressure Sensation with its Adequate Stimulus (Höfler) ......... 26 12. Correlation of Tonal Pitch with Vibration Rate (Höfler) . . 27 13. Correlation of S with R under Weber’s Law (Wundt) . . 29 14. Arrangement of Apparatus for Fechner’s Shadow Experiment . 32 15. Discs for Kirschmann’s Photometer......36 16. Cattell’s Curve .......... 42 17. Curves of Gauss’ Law of Error ....... 43 18. Mechanical Illustration of Cause of Curve of Frequency (Galton) 46 19. Illustration of a Normal Distribution (Galton) .... 48 20. Tuning-forks with Rider Attachments ..... 68 21. Tuning-forks with Screw Attachments ..... 68 22. Galton Bar..........76 23. Sound Pendulum . . . . . . . . .81 24. Delbœuf’s Disc : First Pattern.......88 25. Delbœuf’s Dark Box ....... . .89 26. Delbœuf’s Disc : Second Pattern......89 27. Arrangements of Discs for the Method of Mean Gradations (Wundt) ........... 90 28. Wundt’s Triple Colour Mixer ....... 90 29. Miiller’s Curve of the Distribution of the RL .... 94 XV