Page:Proceedings of the Royal Society of London Vol 4.djvu/99

Rh him a stereoscope, which enables the observer to view the resulting appearances without altering the ordinary adaptation of the eyes, and therefore without subjecting these organs to any strain or fatigue. It consists of two plane mirrors with their backs inclined to each other at an angle of 90°, near the faces of which the two monocular pictures are so placed that their reflected images are seen bv the two eyes, one placed before each mirror, in the same place ; the apparatus has various adjustments by means of which the mag- nitude of the images on the retinae may be varied, and the optic axes differently converged. If the two monocular pictures be thus presented one to each eye, the mind will perceive, from their com- bined effect, a figure of three dimensions, the exact counterpart of the object from which the pictures were drawn ; to show that this curious illusion does not in the least depend on shading or colouring, the illustrations principally employed are simple outline figures, which give for their perceived resultants skeleton forms of three di- mensions. Each monocular outline figure is the representation of two dissimilar skeleton forms, one being the form which it is intended to re- present, and another, which Prof. Wheatstone calls its converse figure Viewed by one eye alone the outline may with equal ease be ima- gined to be either ; but when the two monocular pictures are view^ed one by each eye, the proper or the complemental form may be fixed in the mind ; the former, if the right and left pictures be presented respectively to the right and left eyes ; and the latter, if the right pic- ture be presented to the left eye, and the left picture to the right eye. Many new experiments are then detailed, and a variet}'- of in- stances of false perception of visual objects, some new, others former- ly observed, are traced to these principles ; among others, the well- known apparent conversion of cameos into intaglios. The author next proceeds to show that pictures similar in form but differing in magnitude within certain limits, when presented one to each eye, are perceived by the mind to be single and of intermediate size ; and also that when totally dissimilar pictures, which cannot be combined by the mind into the resemblance of any accustomed objects, are presented one to each eye, they are in general not seen together, but alternately. The memoir concludes with a review of the various hypo- theses which have been advanced to account for our seeing objects single with two eyes ; and the author states his views respecting the influence which these newly developed facts are calculated to have on the decision of this much debated question.

" Experimental Researches in Electricity," Fourteenth Series.

On the general nature and relation of the Electi'ic and Magnetic Forces. By Michael Faraday, Esq., D.C.L., F.R.S., &c. &c.

The author commences by observing that the theory of electrical induction, which he had set forth in the 11th, 1 2th, and 13th se- ries of researches, does not assume or decide anything as to the real nature of the electric forces, but only as to their distribution ; the great question respecting the existence of any electric fluid, or of one, or of two fluids remaining untouched. He then states what