I. - MEASUREMENT. 345 



177O. Transit Instrument, without case. 



Prof. Dr. C. Bruhns, Leipzig, and August Lingkc and Co., 

 Freiberg. 



The transit instrument is to be used in determination of time and of polar 

 altitudes. 



The iron stand is on three feet, and can be set horizontal by means of two 

 levels at right angles to each other. On the stand is a cradle with two 

 supports to cany the telescope, which is always horizontal. There is an 

 arrangement by which this cradle, together with the telescope, can be inverted. 

 In order that the inversion may be as easy as possible, a spring is fixed 

 below the stand, which comes into play when the telescope is inverted, and 

 carries nearly the whole weight of the cradle and telescope. On one side of 

 the cradle is a revolving pivot, which can be brought down between two 

 screws, and according to the position of these screws can be turned through 

 small angles. This movement in Azimuth enables observations to be made 

 with the instrument, not only in the meridian, but also in a vertical arc 

 through the pole, and the index on the cradle which points to a division on 

 the arc gives the position of the cradle in Azimuth. The graduation is so 

 arranged that the interval between two lines gives about 10', and can be read 

 accurately up to whole minutes. 



On the lower stand there is a boss with screws, and when the pivot of the 

 cradle, which rests between the screws, is raised, and the screws at the same 

 time loosened, the telescope can with ease be set in the prime vertical, and by 

 the graduated scale the accurate position in the prime vertical can be read. 



The telescope has an aperture of 73 mm., a focal distance of 80 cm., and 

 two achromatic eye-pieces, with magnifying powers of 60 times and 90 times, 

 also two sun-glasses. The telescope is clamped at half its length, and the 

 clamping apparatus is so arranged that the telescope suffers no pressure. 

 Besides, there is at the middle point of the telescope a support with two 

 screws, in order that the telescope may rest firmly when it is clamped : by 

 this means it has a larger base. In front of the objective is a prism (made 

 by Schroder, of Hamburg, who also supplied the objective) giving total re- 

 flexion, and permitting the same aperture as the objective. The prism is 

 fastened by six screws on the hypothenuse surface against a strong spring ; 

 this spring is so strong that no turning of the prism can occur while altera- 

 tions owing to change of temperature may take place. On one side there are 

 also parts against which the prism is held by two strong screws on the oppo- 

 site side. In the hypothenuse surface of the prism there is a dull spot 

 through which the central illumination of the threads is effected. By a 

 revolving arrangement the illumination can be cut off; this arrangement is 

 particularly simple. 



The telescope carried the head of the eye-piece, and as it can have only a 

 horizontal direction, the eye-piece must remain always in the same position. 

 A micrometer is attached to the eye-piece which can turn through 90 ; this 

 can be used to determine micrometrically the difference of the zenith distance 

 of stars which have nearly the same north and south .zenith distance. 



There are garnets in the bed plates of the supports so as to lessen the 

 friction. For turning the telescope there is a ring of gutta-percha. The 

 telescope carries a level, which can remain permanently on it ; it is graduated 

 to 1"'43. It has also an altitude arc graduated to 10' and reading to 1', by 

 means of a vernier. The Nadir distance can be found by an artificial horizon. 

 One turn of the screw micrometer is = y^th of a revolution, and the pitch of 

 the screw is about th of a Paris inch. 



By this instrument can be determined 

 \ The time in meridian. 



