The Practical Astronomer

achromatic telescope adjusted altitude angle apparent magnitude appear astronomical atmosphere axis beautiful bodies camera obscura candle celestial centre circle colours concave lens concave mirror construction converging convex lens crown glass dark degree direction distant objects distinct distinctly double double stars earth effect equal equatorial exhibits experiment eyeglass eyepiece feet figure fixed flint glass focal length formed glass globe half heavens horizon inches aperture inches diameter inches focal distance instrument invention jects Jupiter lenses likewise luminous magnifying power metal miles minutes moon motion nature nearly object-glass observations optical parallel rays pass perceive phenomena Philosophical placed planet polished principle prism produced proportion radius rainbow rays of light reflecting telescopes reflector represented right ascension ring satellites Saturn seen side solar space spectrum specula speculum stars sun's supposed surface tance tele terrestrial tion tube Venus violet visible vision wheel