The reader will easily perceive that the principle on which Mr. Rogers proposes to construct his telescope is very nearly similar to that of professor Barlow, described above, with this difference, that the correcting lens of the Professor’s telescope is composed of a transparent _fluid_, while that of Mr. Rogers is a _solid_ lens consisting of a convex crown and concave flint. The general object intended to be accomplished by both is the same, namely, to make a correcting lens of a comparatively small diameter serve the purpose of a large disc of flint glass, which has hitherto been very expensive, and very difficult to be procured; and likewise to reduce the length of the telescope while the advantage of a long focal power is secured.--A telescope, on this principle, was constructed 7 or 8 years ago by Mr. Wilson, lecturer on Philosophy and Chemistry, Glasgow, before he was aware that Mr. Rogers had proposed a similar plan. I have had an opportunity of particularly inspecting Mr. Wilson’s telescope, and trying its effects on terrestrial objects with high powers, and was on the whole highly pleased with its performance. It appeared to be almost perfectly achromatic, and produced a distinct and _well-defined_ image of minute distant objects, such as small letters on sign-posts, at 2, 3 and 4 miles distant. But I had no opportunity of trying its effects on double stars or any other celestial objects. The instrument is above 6 feet long; the object lens is a plano-convex of crown glass 4 feet focal distance, and 4 inches diameter, the plain side next the object. At 26 inches distant from the object lens is the compound lens of 2 inches in diameter; and the two lenses of which it is composed are both ground to a radius of 3-3/4 inches. That made of crown glass is _plano-convex_, the other, made of flint glass, is plano-concave, and are placed close together, the convex side being next the object, and the concave side next the eye. The greater refractive power of the flint glass renders the compound one slightly concave in its effect (although the radius of curvature is similar in both), and lengthens the focus to 6 feet from the object-glass; and this is consequently the length of the instrument. The compound corrector so placed intercepts all those rays which go to form the image in the field of view, producing there an achromatic image. The concave power of the corrector renders the image larger than if directly produced by a convex lens of the same focus. The concavity of the corrector is valuable also in this respect, that a very slight alteration in its distance from the object-glass, changes the focal distance much more than if it were plain, and enables us to adjust the instrument to perfect achromatism with great precision. CHAPTER V. ON REFLECTING TELESCOPES. SECT. 1.--HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF THE CONSTRUCTION OF THESE INSTRUMENTS.