. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . psidered, and we sliould abolish all complicated anti-frictionapparatus for the declination axis, retaining it only for the polaraxis to save wear in the teeth of the driving arc. Finally, insteadof making the -finder a short telescope attached to the eye end ofthe instrument, we should give it a focal length equal to that ofthe great object-glass, attaching the cell of its object-glass rigidlyto the cell of the large object-glass and its eye end to the butt endof the main telescope, in order to secure
. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . psidered, and we sliould abolish all complicated anti-frictionapparatus for the declination axis, retaining it only for the polaraxis to save wear in the teeth of the driving arc. Finally, insteadof making the -finder a short telescope attached to the eye end ofthe instrument, we should give it a focal length equal to that ofthe great object-glass, attaching the cell of its object-glass rigidlyto the cell of the large object-glass and its eye end to the butt endof the main telescope, in order to secure the utmost rigidity in therelations of the axes of the two tele- »s scopes. Such a finder would corre-spoud in efficiency to that of theHenry photographic telescope,and would be available as a guid-ing telescope in photographicwork, or for keeping a starexactly on the slit ofspectroscope. The first important in-Btrumenta of type D|Were Mr Lassellsflectors, the largestwhich, and the last,is represented in The polaraxis is sufficiently rigid, butthe long andcomparative-ly slender. Fia 29.—Lassells reflector. forts which carry the pivots of the central craaie are elements ofinstability, especially when the instrument is directed to an objectof considerable hour angle. There is practical confession of thisinstability in the cross-bracing which connects the two forks, andwhich must be removed if the telescope is pointed to an objectbetween the zenith and the elevated pole. The best example of type D is the reflecting telescope of 36-inche3aperture designed by Mr. A. A. Common, with which his exquisitephotographs of nebulae, &c., were made. Theprincipal preliminary conditions which he laiddown as necessary were the following:(1) no tube properly so called, to avoidair-currents in the tube; (2) no mass ofmetal either below or at the side ofthe line joining the large and smallmirrors, to avoid currents frompossible difference of tempera-ture between the mass ofmetal and the
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