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HISTORICAL VIEW OF THE

meter to the telescope : a contrivance on which all thenice determinations of minute distances in modernastronomy depend.

But perhaps the most essential circumstance to theperfection of astronomical instruments, is the adaptationof the telescope to quadrants instead of the plain sights,or small apertures, by which the unassisted eye deter-mined the position of the object observed. Withoutentering into a variety of other causes of superiority(and these were subjects of great discussion at the time),it will amply suffice to mention one. The eye cannotappreciate an angular space in the sky less than aboutthirty seconds. In the best quadrant, with a plain sight,the altitude must he uncertain by that quantity: ! whichis a large error compared with the exact requisitions ofmodern science. If, then, we substitute a telescopemagnifying only thirty times, it will, consequently, en-able us to fix the position to one second.

This plan appears to have been first practised by theFrench astronomer Picard, about 1665; but the ideawas originally started by Gascoigne, before l6'44. Ithas, indeed, been contended, from a letter of Gas-coigne’s, that he had actually brought this method intouse.

Various contrivances were suggested for increasingthe accuracy to which the reading off, as it is termed, ofthe indications of the index on the divided arc of anyastronomical instrument might he carried. That called,from its inventor, the vernier, which has superseded allothers, was suggested in 1631. It is an inventionhardly to be surpassed for refinement of principle andsimplicity of application.

Nearly forty years elapsed, after the first observationof an anomalous appearance in Saturn by Galileo , untilHuyghens , with much more powerful telescopes of hisown construction, discovered that all the appearanceswere no other than those presented by a broad flat ringsurrounding the planet, and viewed under different de-grees of obliquity from the earth. The gradual man-