Popular science monthly . Above: Spiralnebula Messier 101,Ursae Majoris,photographed withthe two - foot re -flector of the YerkesObservatory. Timeof exposure, threehours Another photo-graph of the samenebula, taken withthe sixty-inch re-flector of the Solar Ob-servatory. Time ofexposure, sevenhours, thirty min-utes. Comparisonshows the greaterdetail in thelower photograph deflect the beam of light and form theimage at one side of the tube, where it mayeasily be examined with an eyepiece. Thissecond reflection is accomplished b\- meansof a ])Iane mirror or flat inserted in theupper en
Popular science monthly . Above: Spiralnebula Messier 101,Ursae Majoris,photographed withthe two - foot re -flector of the YerkesObservatory. Timeof exposure, threehours Another photo-graph of the samenebula, taken withthe sixty-inch re-flector of the Solar Ob-servatory. Time ofexposure, sevenhours, thirty min-utes. Comparisonshows the greaterdetail in thelower photograph deflect the beam of light and form theimage at one side of the tube, where it mayeasily be examined with an eyepiece. Thissecond reflection is accomplished b\- meansof a ])Iane mirror or flat inserted in theupper end of the tube and set at an angleof 45°. This flat will neccssarih- cut oftsome of the light falling u|Xin the (principalmirror, but since it is not large and sinceits sujjports are made as slender as possible,there is no serious loss. A Mirror Six Feet in Dia meter At first mirrors were made of speculummetal, an allo\- of copper and tin, whichcan i)e very highly ])olished. As early as1842, the famous reflector of Lord R
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectscience, bookyear1872
