The Encyclopedia britannica; a dictionary of arts, sciences, and general literatureWith new maps, and original American articles by eminent writersWith American revisions and additions, bringing each volume up to date . g the box « contains this on J » ^ of?its supporting tube T a sUr of any declination ..->i. >^ o>«-\-and by combiliin- this motion with rotation of ^ Po- »^^astronomer seated at E is ablo to view any object win tev e in thevisible heavens, except those situated between 10 and 1- hour TELESCOPE 153 ingle. An hour circle attached to EP and a declination circle
The Encyclopedia britannica; a dictionary of arts, sciences, and general literatureWith new maps, and original American articles by eminent writersWith American revisions and additions, bringing each volume up to date . g the box « contains this on J » ^ of?its supporting tube T a sUr of any declination ..->i. >^ o>«-\-and by combiliin- this motion with rotation of ^ Po- »^^astronomer seated at E is ablo to view any object win tev e in thevisible heavens, except those situated between 10 and 1- hour TELESCOPE 153 ingle. An hour circle attached to EP and a declination circleatached to the box containing the mirror N, both of which can beread or set from E, complete the essentials of the instrument. Itsmechanical details present no creat difficulty, andare most conveniently arranged. But we entertainN_ grave doubts as to the practical value cf thex^ instrument, not on mechanical, but on optical, \ grounds. There must be a certain loss of\ ^. ISit from two additional refleiions; but\ \ that could be tolerated for the sake of\ \ other advantages, provided that\ \ the mirrors could be made soS-\ ciently perfect optical planes,*\ A few years ago it was verydiiScult to obtain an op. torn] tically perfect plane6 inches in diameter,and having obtainedit thera remained thefurther diiScuIty ofmounting it so thatin all positions itshould be free from , T . ,. » . , flexure. By makinff i-I«^scoudeeqaator,aL the mirrors^f sUveref glass, one-fourth of their diameter in thickness, JUL Henry havenot only succeeded in mounting them with all necessary rigidityfree from flexure hut- hr-^ given them optically true plane sur-feces, notwithstanding their large diameters, viz., 11 and 157inchtrs. The present writer tested the equatorial coude on doublestars at the Paris observatory in 1SS4, and his last doubts as tothe practical value of the instrument were dispelled. He has neverseen more perfect opticsd deSnition in any of the many telescopeshe has employed, and
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Keywords: ., bookcentury1800, bookdecade1890, bookidencyclopedia, bookyear1892
