That the reader may form a correct idea of the conduct of Mr. Wheatstone in making this claim indirectly, and in a foreign journal, whose editor he has willingly misled, I must remind him that I first saw the reflecting stereoscope at the meeting of the British Association at Newcastle, in the middle of _August 1838_. It is proved by my letter that he and I then conversed on the subject of _prisms_, which at that time he had never thought of. I suggested prisms for displacing the pictures, and Mr. Wheatstone’s natural reply was, that they must be _achromatic prisms_. This fact, if denied, may be proved by various circumstances. His paper of 1838 contains no reference to prisms. If he had suggested the use of prisms in August 1838, he would have inserted his suggestion in that paper, which was then unpublished; and if he had _only once_ tried a prism stereoscope, he never would have used another. On my return to Scotland, I ordered from Mr. Andrew Ross one of the reflecting stereoscopes, and one made with achromatic prisms; but my words do not imply that Mr. Wheatstone was the first person who suggested prisms, and still less that he ever made or used a stereoscope with prisms. But however this may be, it is a most extraordinary statement, which he allows the Abbé Moigno to make, and which, though made a year and a half ago, he has not enabled the Abbé to correct, _that a stereoscope with prisms was made for me_ (or for any other person) _by Mr. Ross_. I never saw such an instrument, or heard of its being constructed: I supposed that after our conversation Mr. Wheatstone might have tried achromatic prisms, and in 1848, when I described my single prism stereoscope, I stated what I now find is not correct, that _I believed_ Mr. Wheatstone had used _two_ achromatic prisms. The following letter from Mr. Andrew Ross will prove the main fact that he never constructed for me, or for Mr. Wheatstone, any refracting stereoscope:— ”2, FEATHERSTONE BUILDINGS, _28th September 1854_. “DEAR SIR,—In reply to yours of the 11th instant, I beg to state that I never supplied you with a stereoscope in which prisms were employed in place of plane mirrors. I have a perfect recollection of being called upon either by yourself or Professor Wheatstone, some fourteen years since, to make achromatized prisms for the above instrument. I also recollect that I did not proceed to manufacture them in consequence of the great bulk of an achromatized prism, with reference to their power of deviating a ray of light, and at that period glass sufficiently free from striæ could not readily be obtained, and was consequently very high-priced.—I remain, &c. &c. “ANDREW ROSS. “To Sir David Brewster.”