586 CEITICAL NOTICES :

(It may be worth mentioning in connexion with sensations of sound that there has just been made for the Leipsic laboratory a tuning- fork vibrating fourteen times per second and giving a distinct note.) Prof. Ladd gives a good account of sensations of light and colour, but is at at times rather brief and does not attempt to decide between rival theories. Lord Eayleigh is not mentioned under either light or sound. There is much need of further research into sensations of light and colour, especially concerning contrast and fusion of sensations. Due prominence is given to sensations of the skin, including the most recent researches of Blix, Goldscheider and Donaldson on the temperature-sense. This too is a field where careful experiment will yield farther im- portant results. Prof. Ladd seems in these chapters to confuse the doctrine of the Specific Energy of the Nerves with the fact that nerves connect special sense-organs and muscles with special brain centres. Chapter v., on " The Quantity of Sensations," contains an ex- cellent discussion of the elaborate researches of Weber, Fechner and others into the relation between the strength of the stimulus and that of the corresponding sensation. I agree with Prof. Ladd in thinking that the " least observable difference" is not a unit with which sensation can be measured. It might also be added that the value is obtained at the " threshold of sensation," just where Weber's generalisation does not hold. The immense amount of research and theorising devoted to " the psychophysi- cal law " has not been wasted, as it has led to more accurate methods of experiment and clearer thinking ; but the positive re- sults seem scant, and may lead us to wonder whether the labour might not have been more wisely distributed. Chapters vi. and vii., on " The Presentations of Sense," give an excellent account of sensation-circles, binocular vision and many other subjects of importance to the psychologist. I am, however, compelled to criticise Prof. Ladd's account of the formation of perceptions. From the translator of Lotze we might have ex- pected a clearer discussion of local signs and the psychological origin of our idea of space. Prof. Ladd says (p. 416) : " It is unnecessary to illustrate in further detail the process by which the mind with its native synthetic activity and with the help of qualita- tively different sensations constructs its field of touch" (Italics mine). But this is just what has been neither explained nor illustrated. Prof. Ladd lays great weight on a distinction between " localisa- tion" and " eccentric projection," seeming almost to think that we have by intuition a knowledge of the shape of our^ own body, and go on thence to construct other things. We may, however, wonder what " eccentric projection " is when we read (p. 387) : " The law of eccentric projection is generally stated thus : Objects are perceived in space as situated in a right line off the ends of the nerve-fibres which they irritate ".