An important discovery for fixing the images produced by the camera--Description of the Daguerreotype process--Preparation of the plate, fixing the impression, &c.--Preparation of photogenic paper--Beneficial effects which this art may produce--Representations of objects in the heavens, &c. _page_ 196-205. CHAPTER III. ON THE OPTICAL ANGLE, AND THE APPARENT MAGNITUDE OF OBJECTS. Various illustrations of the apparent magnitude of objects--Fallacies in relation to apparent magnitudes--Apparent magnitudes in the heavens--Difference between _absolute_ and apparent magnitudes _page_ 206-213. CHAPTER IV. ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES. SECT. 1.--THE GALILEAN TELESCOPE. Construction and peculiar properties of this instrument _page_ 214-217. SECT. 2.--THE COMMON ASTRONOMICAL REFRACTING TELESCOPE. Description of its nature and construction--How its magnifying power is determined. Table of the linear aperture, magnifying powers, &c., of astronomical telescopes from 1 to 120 feet in length--Summary view of the properties of this telescope _page_ 218-224. SECT. 3.--THE AERIAL TELESCOPE. This telescope is used without a tube--Description of the apparatus connected with it, illustrated with figures--Huygens’ Hartsocker’s and Cassini’s large telescopes SECT. 4.--THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL OBJECTS. Arrangement of its lenses--Magnifying power--Manner in which the rays of light are refracted through the telescopes now described _page_ 228-231. SECT. 5.--TELESCOPE FORMED BY A SINGLE LENS. Various experiments in relation to this point--Experiments with a lens 26 focal distance, and 11-1/2 inches diameter _page_ 232-235. SECT. 6.--THE ACHROMATIC TELESCOPE. Imperfections of common refracting telescopes--Dollond’s discovery--Newton’s error--Explanation of the principle of achromatic telescopes--Combination of lenses--Difficulties in the construction of such instruments--Difficulty in procuring large disks of flint glass--Guinaud’s experiments _page_ 235-248. NOTICES OF SOME LARGE ACHROMATIC TELESCOPES ON THE CONTINENT, AND IN GREAT BRITAIN. The Dorpat telescope--Sir J. South’s telescope--Captain Smyth’s--Rev. Dr. Pearson’s--Mr. Lawson’s--Mr. Cooper’s--Mr. Bridges’, &c.,--Achromatics in Cambridge and Paris observatories _pages_ 248-254. ACHROMATIC TELESCOPES OF A MODERATE SIZE, WITH THEIR PRICES, AS SOLD BY LONDON OPTICIANS. The 2-1/2 feet Achromatic--The 3-1/2 feet--The powers applied to it--and the views it gives of the heavenly bodies--The 5 feet achromatic--_Stands_ for telescopes, illustrated by engravings _page_ 254-264. PROPORTIONS OR CURVATURE OF THE LENSES WHICH FORM AN ACHROMATIC OBJECT-GLASS. Various tables and explanations _page_ 265-269. ACHROMATIC TELESCOPES COMPOSED OF FLUID LENSES.