The important subject of which we are treating has been discussed by Aguilonius with singular ingenuity ; and his observa-tions are so interesting, that we shall give them in his own worus. "When one object," he says, " is seen with two eyes, the angles at the vertices of the optical pyramids (viz. H a F, g B e) are not always equal; (they are equal in the vision of a sphere and a cylinder;) for beside the direct view, in which the pyramids ought to be equal, into whatever directions both eyes are turned they receive pictures of the objects under unequal angles, the greater of which is that which is terminated at the nearer eye, and the lesser that which regards the remoter eye. This, I think, is perfectly evident; but I consider it as worthy of admiration, how it happens that bodies seen by both eyes are not all confused and shapeless, though we view them by the optical axes fixed on the bodies themselves. For greater bodies seen under greater angles appear lesser bodies under lesser angles. If, therefore, one and the same body which is in reality greater with one eye, is seen less on account of the inequality of the angles in which the pyramids are terminated, the body itself must assuredly be seen greater or less at the same time, and to the same person that views it; and therefore, since the images in each eye are dissimilar (minime sibi congruuni), the representation of the object must appear confused and disturbed (confusa ac perturbata) to the primary sense." In order to understand this passage, we must state, as a well-known fact, that in binocular portraits the distance between the tip of the nose and the tip of the ear is greater in the one picture than in the other, and consequently the line joining these points subtends a greater angle in the one than in the other. When these two lines, therefore, are combined. Aguilonius concludes that the virion of the tip of the note and the tip of the ear must be confused, as the ends of the lines cannot be united.
"This view of the subject." he continues, "is certainly consistent with reason; but what is truly wonderful is, that it is not correct, for bodies are seen clearly and dis-tinctly with both eyes when the optic axes are converged upon them. The reason of this, I think, is, that the bodies do not appear to be single, because the apparent images which are formed from each of them in separate eyes exactly coalesce (sibi mutuo exacte congruunt), but because the common sense imparts its aid equally to each eye, exerting its own power equally in the same manner as the eyes are converged by means of their optical axes. Whatever body, therefore, each eye sees with the eyes conjoined, the common sense makes a single notion, not composed of the two which belong to each eye. but belonging and accommodated to the imaginative faculty to which it (the common tense) assigns it."
Now though the explanation here given of the distinct appearance of the solid composed of two dissimilar pictures is not correct, yet Aguilonius clearly asserts the second truth, that though the unequal lines and angles do not coalesce, yet the body is seen distinctly and in its true solidity, in consequence of the combination of the two pictures of it as seen by each eye.
From these details it is manifest that the two fundamental truths on which the Stereoscope depends were well known to Aguilonius and others ; and that nothing more was wanted than a method of forming two dissimilar pictures of objects, and a method of uniting them when formed.
Upwards of thirty years ago, Mr. Elliot, now a teacher of mathematics in Edinburgh, was led to study the subject of binocular vision, in consequence of having writter an essay in 1823. for the Logic class, "On the means by which we obtain our know-ledge of distances by the eye." From that, time he was familiar with the idea that the relief of solid bodies when seen with both eyes was produced by the union of the two dissimilar pictures of them as seen by each eye, which he believed was known to every student of vision. During the year 1834, or previous to it, he had resolved to make an instrument for uniting two dissimilar pictures, or of constructing a stereoscope. But though he had invented the instrument, he delayed its construction till 1839, when he was asked to write a paper for the Polytechnic Society in Liverpool. The instrument was exhibited to Mr. Richard Adie, optician, and Mr. G. Hamilton, lecturer on chemistry ; hut owing to the diffi - culty of obtaining binocular pictures for it, he proceeded no further with his invention. In order, however, to show the effect of the instrument to his friends, he constructed a rude picture of a landscape, as seen by each eye separately; and when these two pictures were placed in his instrument, the parts of the landscape appeared at different distances from the eye, or in their true relief. As this was undoubtedly the first landscape constructed for, and seen in relief through the Stereoscope, it possesses much interest; and we have given an accurate copy of the dissimilar pictures in the an-nexed diagram, as they were placed by Mr. Elliot, at the farther end of a box 18 inches long, 7 broad, and 4 1/2 deep. In their present position they will appear in relief when united by the Steroscope, or by converging the optic axes to a point at a proper distance beyond them. Had photography been in existence, to enable Mr. Elliot to obtain binocular pictures of landscapes and other objects, the application of the Stereoscope to natural scenery and to portraiture would not have been so long delayed.
In the month of August, 1838, Mr. Wheat-stone exhibited an instrument, under the name of the Reflecting Stereoscope, to the British Association which met at Newcastle; and an account of it was published in the Philosophical Transactions for that year. The merit of this invention belongs exclusively to Mr. Wheatstone, and nobody has either directly or indirectly laid clam to it Although it answers the purpose for which it was contrived, it is a clumsy and bulky apparatus, unnecessarily expensive, and now seldom seen. The binocular representations •which it raised into relief were chiefly those of geometrical solids ; but the idea of applying it to landscapes or portraits is never once mentioned in his paper. The theory of the instrument, as given by Mr. Wheat-stone, was shown to be incorrect by the writer of this article, who first gave the true theory in the Edinburgh Transactions for 1843; and in the experiments which he made on the subject, he was led to the construction of several new stereoscopes, but particularly to the Lenticular Stereoscope now in universal use.
"The Reflecting Sterescope of Mr. Wheat-stone was at this time," as the Abbe Moigno remarks, "almost completely forgotten." Its merits had never been sufficiently under-Stood ; and even the Lenticular Stereoscope, after photography had supplied it with binocular portraits, excited a very limited interest. 1 offered it gratuitously to opticians in London and Birmingham; but it was not till the year 1850, when I took one to Paris, and showed it to the Abbe Moigno and M. Duboseq, that it was appreciated and brought into notice. Having executed a number of binocular pictures of statues and bas-reliefs, and portraits of celebrated individuals, M. Duboseq, to use the words of Abbe Moigno, " showed the wonderful effects of the instrument to natural philosophers and amateurs, who flocked to him in crowds, and from whom they elicited a spontaneous and unanimous cry of admiration."
In the noble collection of philosophical instruments displayed by M. Duboseq in the Great Exhibition of 1851, he placed a Lenticular Stereoscope, with a set of binocular pictures in daguerreotype. The instrument attracted the particular attention of the
Queen, and in a short time M. Duboseq received many orders for stereoscopes from England and the United States.
Such is a brief history of the Lenticular Stereoscope, and of its introduction into this country. It is now in general use over the whole world, and it has been estimated that more than half a million of the Lenticular Stereoscopes have been sold. Photographers are employed in every part of the globe in taking binocular pictures for the instrument, - among the ruins of Pompeii and Herculaneum - on the glaciers and in the valleys of Switzerland - among the public monuments in the Old and New World - in the museums of ancient and modern life - and in the sacred precincts of the domestic circle.