TRANSACTIONS OF SUB-SECTION TI. 688 
answers are given; and again as the 80 per cent. point; and 20 per cent., or any 
per cent. might be chosen. Especially is it misleading to say that stimuli below 
the average threshold cannot be distinguished from one another. On any one 
occasion anything may happen, but when a number of judgments are made the 
result as in Table 2 indicaces differences between all the stimulus values used. 
Similar records exist for many thresholds, in particular for the difference 
threshold for lifted weights. Those therefore who, like Poincaré,’ say that 
weights which differ by less than the ‘difference threshold’ produce identical 
sensations, are wrong, and the arguments based on this statement are incorrect. 
7. A Simple Method of demonstrating Weber’s Law and its Limitations. 
By Suepuerp Dawson, M.A., B.Sc. 
The main facts of Weber’s Law with its limitations can be demonstrated easily 
and quickly in the following manner :— 
Along a radius of a disc of white paper pierce a number of circular holes of 
equal diameter. Punch equal and similarly placed holes in other discs of grey 
paper of various intensities. Rotate one of these discs in front of a box lined 
with black velvet, and a series of dark rings will be formed on the disc which 
differ progressively in brightness, the innermost being the darkest. The size 
of the holes should be such that the smallest ring can be easily seen while the 
largest is indistinguishable: holes from three to four millimétres in diameter 
and about 1°5 cms. apart on a disc 13 cms. in diameter give satisfactory results. 
If no light be reflected through the holes in the disc, then the ratios of the 
intensities of the rings to the intensity of the disc depend only on the diameter 
of the holes and the distances from the centre. Hence, if all the discs be 
similarly cut and Weber’s Law be true for brightnesses, the same number of 
rings should be distinguishable on each disc. To demonstrate the law we have, 
therefore, only to rotate the disc and count the number of rings that can be 
seen. 
As the intensities of the grey papers can be found by matching them with 
mixtures of black and a standard white, and as the ratio of the intensity of each 
ring to the intensity of the background can be found by measuring the diameter 
of the hole and its distance from the centre, it is possible to state the results 
numerically in terms of thresholds of discrimination and to show to what extent 
Weber’s Law is valid. 
8. Some Experiments on Recovery from Fatigue. 
By Miss May Smiru. 
1. The aim of this set of experiments. 
2. Description of the apparatus and details of the experiments. 
3. Results :— 
(a) Average errors for each week and consideration of the kinds of 
errors. 
(d) Pas slack of fatigue induced by three nights with very little 
_ sleep. 
(c) The return to normal. 
(d) A comparison of subjective and objective records. 
4. Some general conclusions from these experiments. 
9. A Comparative Investigation of Fatigue Tests. By J. H. Wivns. 
* vy. Urban, op. cit., and especially Fernberger, Psychol. Monographs, 1913, 
xiv. (4), 54, 64, where much smaller steps are used. 
7 Science and Hypothesis, Scott, 1905, 22. 
