258 Collimation Error of Astronomical Instruments. [May, 



III. — Collimation Error of Astronomical Instruments. By J. G. Taylor, 

 Esq. H. C. Astronomer, Madras. 

 Ten years have now elapsed since Captain Kater's plan for deter- 

 mining the position of the line of collimation by means of a floating 

 Collimator was brought before the public, and his ingenuity rewarded 

 by the gold medal of the Royal Astronomical Society. It has happened, 

 however, with this, as with many other great and good inventions, 

 which are true in theory, that the application to practice is attended 

 with so much uncertainty, as almost completely to render the plan 

 unavailable ; hence it is, that the results of observations made with 

 the assistance of the floating collimator (if any there bej have never yet 

 been made public. I offer these remarks with a view of saving the 

 amateur astronomer from the vexatious disappointments which he may 

 expect to meet with in the employment of the floating collimator ; and, 

 at the same time, of offering a plan to supersede its use, which is 

 totally free from any sort of uncertainty : and can, moreover, be applied 

 with much greater facility than the floating collimator ; the plan in 

 question consists of making the telescope a collimator to itself, by viewing 

 the image of the wires reflected from a basin of quicksilver, at the same 

 time that the direct image is viewed in the ordinary way through the 

 eye-piece ; to accomplish this, it is only necessary to exhibit a bright 

 light behind the wires, so as not to interfere with the eye of the observer 

 when applied to the eye-piece — in the case of the Madras Mural 

 Circle, to which this principle was lately applied, I introduced a plain 

 silver speculum into the eye-piece of the telescope between the eye^ 

 o-lass and the wires, having its polished surface directed towards the 

 wires; the speculum was suspended in the cell of the eye-piece by two 

 screws, allowing it to revolve on them as an exis, and was furnished 

 with a small hole in the centre, through which the wires in the tele- 

 scope could be seen ; the telescope being now directed to the nadir to 

 a basin of quicksilver, the speculum was turned on its axis until a ray 

 of light (admitted through a hole about -^o °f an mcn diameter, drilled 

 in the side of the telescope), was reflected from it, and made to fall per- 

 pendicularly upon the wires (an operation occupying about five minutes 

 to adjust, and not afterwards requiring alteration), by this means, in 

 addition to the ordinary direct image of the horizontal wire, a reflected 

 image was obtained, situated as much to the north of the nadir as the 

 other was to the south, and vice versa ; nothing more was necessary 

 now than to clamp the circle and bring the wire to cover its reflected 

 image by the tangent screw, when the reading gave (the circle being 

 adapted to measure north polar distance) 180°-|- colat. -f. E ; subtract- 

 ing the two former or 256° 55' 50'' E., the error of collimation, became 

 known. Since establishing the above mode of observation, which I 



