CHAPTER VI

INSTRUMENTAL ADVANCES

Two Principles of Telescopic Construction--Early Reflectors--Three Varieties--Herschel's Specula--High Magnifying Powers--Invention of the Achromatic Lens--Guinand's Optical Glass--The Great Rosse Reflector--Its Disclosures--Mounting of Telescopes--Astronomical Circles--Personal Equation 108

## PART II

_RECENT PROGRESS OF ASTRONOMY_

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