652 TRANSACTIONS OF SECTION I. 
mutual influence of the irritations on their intensity, their clearness and location. 
These effects are bound to a certain proximity of the stimuli, and wane with 
their growing distance. There is, for instance, no effect from one hand upon the 
other, but a pronounced one from one finger upon the next or from a point on 
the forearm upon any one on the same segment. 
By comparing a stimulus H given on a finger, say, of the left hand, with 
another, V, following the first in two seconds on the right, it is possible by 
adjusting the intensity of the latter to make both equal. If now together with H 
a stimulus N is given on an adjoining finger of the left hand, the succeeding 
stimulus V appears to the subject much too weak in comparison with H. To 
make V again equal with H, its intensity must be increased, according to the 
intensity of N, by 80 per cent. of the original value. 
The subject moreover states that the stimulus H, if accompanied by N, loses 
much of its clearness or distinctness, so that, in comparing it with V, it is 
difficult to arrive at a definite opinion as to its strength. According to the sub- 
ject, the stimulus H acquires under these circumstances a dim, blunt, or blurred 
character. By repeating the irritation and directing the subject to concentrate 
his attention as far as possible upon H, the difficulty may be largely overcome. 
Finally, there is always a tendency of two simultaneous irritations to attract 
each other, that is to say, the subject is liable to underrate the distance between 
them, though he can judge it tolerably well, if the two stimuli are given suc- 
cessively. The deception is so cogent that it is not overcome by the subject 
being acquainted with the real position of the irritated spots on the skin. If 
the stimuli are subjectively of equal strength, the dislocation of the correspond- 
ing sensations is the same for both, and they appear therefore equally displaced 
towards the mid-point of the line which connects the spots on the skin. If the 
stimuli are of different intensity, the weaker is always drawn towards the 
stronger one. 
The author expressed the opinion that these mutual influences sufficiently 
explain the well-known difference between the successive and simultaneous space 
threshold or ‘Raumschwelle.’ He entered into a discussion of the processes in 
the central nervous system by which the described effects may be brought about. 
7. Binocular and Uniocular Discrimination of Brightness. 
By SurpHerp Dawson, M.A., B.Sc. 
The object of this investigation was the determination and explanation of 
any differences there may be between uniocular and binocular vision in detecting 
small differences of brightness. The stimulus used was a grey ring on a brighter 
background, which was formed by rotating in front of a well-lighted, milk-glass 
screen a sector fitted with a strip which could be placed at any distance from 
the centre and made to project to any required extent. 
The observations of four trained observers whose vision is normal show that 
the power of discriminating is better when both eyes are used than when the 
better eye is used alone, that the subjects are more certain in their judgments, 
that the curve representing the correct binocular judgments falls more rapidly 
than that of the uniocular judgments, and that the ring is located more quickly 
and more accurately. 
Material for the explanation of these results is found in the descriptions of 
the ring and of the processes of finding and locating it. The binocular image 
appears to be due to the psychical fusion of the uniocular images, and in these 
experiments differs from them most in respect of steadiness. That there is 
probably little or no difference in brightness is demonstrated by the difference 
jn gradient of the curves representing the number of correct judgments at the 
different intensities (which would necessarily be the same if the difference were 
only one of brightness), by the occasional detection of as small intensity-differences 
by one eye as by two, and by the absence of any perceptible difference between 
the binocular and uniocular images when the ring is clearly perceptible. The 
difference in steadiness is due to the fusion in the binocular image of sensa- 
tions which are intermittent in their appearance, the result of the fusion 
being a reduction of the periods of intermittence. This explanation is sup- 
ported, not only by measurements of the periods of intermittence and direct 
