£ TRANSACTIONS OF THE SECTIONS. 7 
a 
that is, place the drawings so that the average distance of similar parts in each is 
equal to twice that quantity. If this is not correctly done, the eye of the observer 
will correct the error, by making the images coalesce, without being sensible that it 
is making any such effort. When the dissimilar drawings are thus united, the solid 
will appear standing, as it were, in relief, between the two plane representations of 
it. In looking through this stereoscope, the observer may probably be perplexed 
by the vision of only the two dissimilar drawings. This effect is produced by the 
strong tendency of the eyes to unite two similar, or even dissimilar drawings. No 
sooner do the refracted images emerge from their respective drawings, than the eyes, 
in virtue of this tendency, force them back into union; and though this is done by 
the convergency of the optic axes to a point nearer the eye than the drawings, yet 
the observer is scarcely conscious of the muscular exertion by which this is effected. 
This effect, when it does occur, may be counteracted by drawing back the eyes from 
the lenses, and shutting them before they again view the drawings. While the semi- 
lenses thus double the drawings and enable us to unite two of the images, they at 
the same time magnify them, —an advantage of a very peculiar kind, when we wish 
to give a great apparent magnitude to drawings on a small scale, taken photogra- 
phically with the camera. The lenticular stereoscope may be made of any size. 
_ Sir David Brewster then described how we may see at the same time a raised anda 
hollow cone, the former being produced by the union of the first with the second, and 
the latter by the union of the second with the ¢hird figures. This method of exhi- 
biting at the same time the raised and the hollow solid, enables us, he said, to give 
an ocular and experimental proof of the usual explanation of the cause of the large 
size of the horizontal moon, of her small size when in the meridian at a considerable 
altitude, and her intermediate apparent magnitude at an intermediate altitude. As 
the summit of the raised cone appears to be nearest the eye of the observer, the 
summit of the hollow cone furthest off, and that of the flat drawing on each side at 
an intermediate distance, these distances will represent the apparent distance of 
the moon in the zenith of the elliptical celestial vault, in the horizon, and at an alti- 
tude of 45°. The circular summits thus seen are in reality exactly of the same size, 
and at the same distance from the eye, and are therefore precisely in the same 
circumstances as the moon in the three positions already mentioned. If we now 
contemplate them in the stereoscope, we shall see the circular summit of the hollow 
cone the largest, like the horizontal moon, because it seems at the greatest distance 
from the eye ; the circular summit of the raised cone the smallest, because it appears 
at the least distance, like the zenith moon; and the circular summit of the cones 
on each of an intermediate size, like the moon at an altitude of 45°, because their 
distance from the eye is intermediate. No change is produced in the apparent 
magnitude of these circles by making one or more of them less bright than the 
rest, and hence we see the incorrectness of the explanation of the size of the hori- 
zontal moon, as given by Dr. Berkeley. When the observer fails to see the object in 
- relief from the cause already mentioned, but sees only the ¢wo drawings, if there are 
two, or the three drawings, if there are three, the plane of the drawings appears 
deeply hollow ; and, what is very remarkable, if we look with the eccentric lenses ata 
- flat table from above, it also appears deeply hollow, and if we touch it with the palm 
of our hand, i¢ is felt as hollow, while we are looking at it, but the sensation of 
~ hollowness disappears on shutting our eyes. Sir David Brewster described a variety 
_ of forms in which he had constructed the stereoscope, by means of lenses, mirrors 
and prisms. ‘The sense of sight, therefore, instead of being the pupil of the sense 
of touch, as Berkeley and others have believed, is, in this as in other cases, its 
teacher and its guide. Sir D. Brewster’s simplified stereoscopes may not only be 
_ rendered portable, but may be constructed out of materials which every person pos- 
__Sesses, and without the aid of an optician. A fuller account of these instruments 
will be found in the forthcoming volume of the Transactions of the Royal Scottish 
Society of Arts. 

é Experiments on the Inflection of Light. By Lory Broucuam, F.R.S.— 
oA communication from Lord Brougham was read by Sir David Brewster. His 
Lordship’s experiments were made at his seat at Cannes in Provence, with a very fine 

