14 REPORT—1858. 
found to be the very best that has yet been constructed for the purposes of photo- 
raphy. 
7 Sir David Brewster gave a brief analysis of this long and interesting communica- 
tion, of which the following is the conclusion :—‘‘ What I have offered is a kind of 
universal instrument, consisting of three achromatic lenses. The front lens, in con- 
nexion with a little larger posterior lens, and mounted like an ordinary portrait lens, 
reproduces a strongly illuminated and well-defined picture of the usual size. The 
same front lens, in connexion with a smaller posterior lens, reproduces a large picture 
with perfect perspective, equal distribution of light, and equal sharpness of the pic- 
ture. It forms a combination for multifarious applications, for taking views, groups 
of persons, maps,” &c. 
On an Apparatus for exhibiting Optical Illusions of Spectral Phenomena. 
By Henry Drrcks. 
The author, after quoting some passages in Sir David Brewster’s ‘ Natural Magic,’ 
in which the author had intimated that reflexion by concave specula must form the 
basis of all spectral illusions by reflexion, and pointing out the inconvenience of 
using these for producing images of living and moving persons, in consequence of 
their inverting objects, stated that he had contrived a means by which living actors, 
some the real persons, others the images of persons concealed from the direct view 
of the spectators, might be formed by a large plate of glass dividing the room in 
which the exhibition was made, the spectators being in a darkened portion above, 
but at one side of the glass plate; while the living persons on the other side of it 
could be seen quite clearly through the glass, and the images of other persons, walk- 
ing about in the room under them, seen by reflexion, would appear in the same place 
as the living persons seen directly, such arrangement becoming, in fact, a transparent 
mirror, and the actors could be thus made to appear to perform most amusing spec- 
tral feats, such as passing through walls, into and coming out of the living actors, 
and soon. 
On a New Case of Binocular Vision. By the Rev. J. Dincve. 
The author remarked, that without some provision imperfect vision would conti- 
nually arise from the difference of the pictures in the two eyes. Even in the same 
object, the point looked at is often visible only to one eye, and the picture of it is 
combined in the sensorium with the picture of another part; and in other cases the 
superposition of different images would sometimes lead to great inconvenience and 
confusion. It sometimes happens, for instance, that in looking at a field of view at 
some distance, objects considerably nearer are so interposed as to present themselves 
in the picture formed in one eye and not in the other. Thus, in looking at a landscape, 
if the finger or any other object is held before one eye, the image of it from the one 
retina is superposed in the sensorium on a part of the landscape formed in the other 
eye. On mere physical principles, this might be expected to blot out or greatly con- 
fuse that part of the landscape upon which it was placed ; but upon trial this is not 
found to be the case, as that part is merely a little dimmer than the rest from being 
seen only with one eye, but is equally distinct and as truly coloured. By various 
experiments the author had ascertained that this was the result of a peculiar power 
of the will, by means of which the mind is enabled, when two different images are 
superposed in the sensorium, to select whichever it pleases, to bring that object into 
view, and entirely to obliterate the other,—it sees, in fact, whichever it wills to see, 
and the other image, simply by being neglected, becomes invisible. In ordinary 
vision, the determination of the image to be seen is effected by the same act of the 
will which determines the position of the optic axes; but by certain arrangements 
which were indicated, both images may be made to have the same relation to the 
optic axes ; and as the predisposition to select one or the other is thus obviated, it is 
made indifferent to the mind which of the two images that occupy the same place in 
the sensorium it shall see*. When these arrangements are made, it is found that 
* The proof of the law might be allowed to rest on an accurate observation of what is 
almost constantly taking place in the act of vision; but the following simple experiment may 
a 
