Chapter VI

MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES.

1. Adjustments requisite to be attended to in the use of telescopes--2. State of the atmosphere most proper for observing terrestrial and celestial objects--Average number of hours in the year fit for celestial observations.--3. On the magnifying powers requisite for observing the phenomena of the different planets--Comets--Double stars, &c.--Illustrated at large from p. 369-380.--4. Mode of exhibiting the solar spots--Eye-pieces best adapted for this purpose--How they may be exhibited to a large company--Mode in which their dimensions may be determined.--5. On the _space-penetrating_ power of telescopes--Herschel’s observations on space-penetrating powers--Comparison of achromatic and Gregorian reflectors.--6. On choosing telescopes, and ascertaining their properties--Various modes of ascertaining the goodness of telescopes--General remarks and cautions on this point--A circumstance which requires to be attended to in using achromatics.--7. On the mode of determining the magnifying power of telescopes--Various experiments in relation to this point.--8. On cleaning the lenses of telescopes

_page_ 361-407.

ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS.

Mode of adapting a telescope for this purpose--objects to which they may be applied

_page_ 407-411.

REFLECTIONS ON LIGHT AND VISION, AND ON THE NATURE AND UTILITY OF TELESCOPES.

Wonderful and mysterious nature of light--The organ of vision, and its expansive range--Wonderful nature of the telescope, and the objects it has disclosed to view--No boundaries should be set to the discoveries of science and the improvement of art--The telescope is a machine which virtually transports us to the distant regions of space--It enlarges our views of the sublime scenes of creation--It has tended to amplify our conceptions of the empire and the attributes of the Deity--Various uses of this instrument in relation to science and common life

_page_ 411-431.

CHAPTER VII.

ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND SPECULA.

1. Directions for grinding lenses for eye-glasses, microscopes, &c.--2. Method of casting and grinding the specula of reflecting telescopes--Compositions for speculum metal--To try the figure of the metal--To adjust the eye-hole of Gregorian reflectors--To center the specula--To center lenses. _page_ 432-442.

PART III.

ON VARIOUS ASTRONOMICAL INSTRUMENTS.

CHAPTER I.

ON MICROMETERS.

Various descriptions of micrometers--_Cavallo’s_ micrometer described--To ascertain the value of its divisions--Practical uses of this micrometer--Problems which may be solved by it--Tables for facilitating its use

_page_ 443-452.