396 SEC. ll.^-ASTEONOMT. 



When the instrument is reversed, which can be very quickly effected, 

 the level and lamp remain hanging, and the clamping-screw need not be 

 loosened. There is a micrometer at the eyepiece. 



1769. Photograph of Transit Instrument, with straight 

 telescope of 3 3" aperture. A. Rcpsold and Sons, Hamburg. 



In reversing it is not necessary to loosen the clamping-screw, and th 

 level remains in position. There is a micrometer at the eyepiece. 



1770. Transit Instrument. 



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

 Freiberg. 



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

 tudes in the prime vertical. 



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 carry 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 m the meridian, but also in a vertical arc 

 through the polestar, 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 sci-ews, is raised, and the screws at the same 

 time loosened, the telescope can with ease be set ia 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 eyepieces, 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- 

 flection, and giving 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 Avhich 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 regulated ; this arrangement 

 is particularly simple. 



The telescope carries 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. 



