. Lighthouse construction and illumination; . he constructionof totally reflecting prisms in the following manner:— M iST (Fig. 138) is the ruler marked, as already described,for an index of refraction of 15. F C is the direction of a ray from the flame whichhas to be bent intothe direction C sides of theprism C Q and C Pmust have such di-rections that the rayr C falling on theside C P at thepoint C will be re-fracted into the di-rection C Q, and C Pmust be such thatthe ray E C tracedi backward must, after falling on C Q, be refracted in the direction C P. This isfound by the instrument
. Lighthouse construction and illumination; . he constructionof totally reflecting prisms in the following manner:— M iST (Fig. 138) is the ruler marked, as already described,for an index of refraction of 15. F C is the direction of a ray from the flame whichhas to be bent intothe direction C sides of theprism C Q and C Pmust have such di-rections that the rayr C falling on theside C P at thepoint C will be re-fracted into the di-rection C Q, and C Pmust be such thatthe ray E C tracedi backward must, after falling on C Q, be refracted in the direction C P. This isfound by the instrument shown. The dotted line throughC L bisects the angle E C E, and the point L is moved to orfrom the point C on the line C L, until it is observed thatthe end of the ruler M N just touches the line F C produced,while the mark 15 coincides with the line C Q. The linesC Q and C P then form the two refracting sides of the prism;the reflecting side is drawn, so that all the rays falling uponit are reflected in directions parallel to the line C Fi;C. 13S. Mr. Brehners Optical Protractor. — This instrument, asshown on Plate XXV., consists of two parts : the lower limbL H N G, which forms a complete circle, and the upper limb. MISCELLANEOUS SUBJECTS. 243 A K B, Mliicli is a semicircle. The lower limb is made tomove easily roimd a centre on the upper limb, which is per-forated and filled in with glass, and two axial cross lines aredrawn on the glass. The under side of this centre part restson the table, and is fitted with three points which penetratethe table and keep the upper limb from turning; while thelower one, which is next the table, turns freely. One half ofthe lower limb is divided into degrees, and is numbered fromthe middle towards each side, and also from each side towardsthe middle; the opposite half is so divided as to make thesines of the opposing angles have a ratio of 151 to 1. For finding the path of a ray through a piece of glass :Let Gr H represent the surface
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Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
