With a good achromatic prism the single-prism stereoscope is a very fine instrument; and no advantage of any value could be gained by using _two achromatic prisms_. In the article on New Stereoscopes, published in the Transactions of the Royal Society of Arts for 1849, and in the Philosophical Magazine for 1852, I have stated in a note that _I believed_ that Mr. Wheatstone had used _two achromatic prisms_. This, however, was a mistake, as already explained,[41] for such an instrument was never made, and has never been named in any work previous to 1849, when it was mentioned by myself in the note above referred to. [41] See Chap. i. pp. 33-36. If we make a double prism, or join two, as shewn at P, P′ in Fig. 38, and apply it to two dissimilar figures A, B, one of which is the reflected image of the other, so that with the left eye L and the prism P we place the refracted image of A upon B, as seen by the right eye R, we shall see a _raised_ cone, and if with the prism P′ we place the image of B upon A we shall see a _hollow_ cone. If we place the left eye L at O, behind the common base of the prism, we shall see with one-half of the pupil the _hollow_ cone and with the other half the _raised_ cone. [Illustration: FIG. 38.] 6. _The Opera-Glass Stereoscope._ As the eyes themselves form a stereoscope to those who have the power of quickly converging their axes to points nearer than the object which they contemplate, it might have been expected that the first attempt to make a stereoscope for those who do not possess such a power, would have been to supply them with auxiliary eyeballs capable of combining binocular pictures of different sizes at different distances from the eye. This, however, has not been the case, and the stereoscope for this purpose, which we are about to describe, is one of the latest of its forms. [Illustration: FIG. 39.] [Illustration: FIG. 40.] In Fig. 39, MN is a small inverting telescope, consisting of two convex lenses M, N, placed at the sum of their focal distances, and OP another of the same kind. When the two eyes, R, L, look through the two telescopes directly at the dissimilar pictures A, B, they will see them with perfect distinctness; but, by the slightest inclination of the axes of the telescopes, the two images can be combined, and the stereoscopic effect immediately produced. With the dissimilar pictures in the diagram a _hollow_ cone is produced; but if we look at B with the telescope M′N′, as in Fig. 40, and at A′ with O′P′, a _raised_ cone will be seen. With the usual binocular slides containing portraits or landscapes, the pictures are seen in relief by combining the right-eye one with the left-eye one. The instrument now described is nothing more than a double opera-glass, which itself forms a good stereoscope. Owing, however, to the use of a concave eye-glass, the field of view is very small, and therefore a convex glass, which gives a larger field, is greatly to be preferred.