A manual of spherical and practical astronomy, embracing the general problems of spherical astronomy, the special applications to nautical astronomy, and the theory and use of fixed and portable astronomical instruments, with an appendix on the method of least squares . s the reflector. A rayfrom the central mirror entering one of the perpendicular facesis totally reflected at the inner face and passes out through the * Special instruments for measuring the dip of the sea horizon have been an account of Troughtons Dip-Sector, see Simmss Treatise on MathematicalInsfrumentt. 128 PR
A manual of spherical and practical astronomy, embracing the general problems of spherical astronomy, the special applications to nautical astronomy, and the theory and use of fixed and portable astronomical instruments, with an appendix on the method of least squares . s the reflector. A rayfrom the central mirror entering one of the perpendicular facesis totally reflected at the inner face and passes out through the * Special instruments for measuring the dip of the sea horizon have been an account of Troughtons Dip-Sector, see Simmss Treatise on MathematicalInsfrumentt. 128 PRISMATIC CIRCLE. other perpendicular face in the direction of the sight line of thetelescope. The height of the prism is only one-half the diameterof the object lens of the telescope, and therefore direct raysfrom any object passing over the prism enter the telescope andare brought to the same focus as the reflected rays. When thecentral mirror is parallel to the longest side of the prism, as inFig. 32, two images of the same object are in coincidence, andthe index correction is determined as in the sextant, except thatevery reading is here the mean of the readings of the twoverniers. Now revolving the index into the position, Fig. 33, an object Fig. 32. Fig.
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