of Edinburgh, Session 1870-71. 361 
When we introduce a blue strip at the one side of the stereo- 
scope, and a red or yellow one at the other side, till they appear to 
overlap or unite into one object, the result is increased brightness 
where they overlap ; but there is no blending of the colours so as 
to produce purple or green. The one coloured strip, as in the 
experiment with the coins, shines through the other; or at one time 
the colours are alternately visible, at another time one-half of each 
coloured end only is visible, and occasionally spots of the one are 
seen to shine through the ground colour of the other, thus estab- 
lishing the important fact or law, that though the combination of 
different colours, external to the living organism, produces the 
effect of an intermediate colour, yet the impulse of different colours 
on separate retinae can not be so combined by the mind, but the 
impulse peculiar to each colour is conveyed by the nerve receiving 
it to the sensorium unchanged, and excites in the mind its own 
characteristic sensation. The increased intensity where the adjoin- 
ing nerve-fibres in the sensorium are all in action I attribute to the 
well-known law of irradiation, or lateral expansion of nervous 
action, which exists among neighbouring nerve-fibres when power- 
fully excited 
The arrangement of the fibres above suggested explains — 
ls£. The nature and cause of the peculiar action of the identical 
retinal points. 
2d. The physical cause of single and double vision. 
3 d. The reason why we have increased brightness by the use of 
both eyes, whether in ordinary vision or when using the stereoscope. 
Ath. The several phenomena force us to the conclusion that visual 
sensation is notin the retinas, but in a common cerebral sensorium. 
3. Additional Note on the Motion of a Heavy Body along 
the Circumference of a Circle. By E. Sang, Esq. 
Abstract. 
In the course of physical inquiries we meet with many problems 
having the appearance of great simplicity, and yet presenting to 
the analyst difficulties of the highest order. The law of the 
motion of a heavy body along the circumference of a circle is one 
of these. 
