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.

Blair’s fluid telescope, with an account of its performance--Barlow’s large refracting telescope with a fluid concave lens--Its construction, and the effect it produces on double stars, &c.--Rogers’ achromatic telescope on a new plan--Wilson’s telescope, &c.

_page_ 269-283.