of the large Refracting Telescope just completed. +7 



fixed, and a print of which accompanies the present descrip- 

 tion *. 



When the instrument was sent off to Dorpat, the microme- 

 ter, &c. belonging to it were not completed ; but they will be 

 there before the instrument can be entirely set up. 



In the line micrometer belonging to the instrument, both 

 threads may be separately moved by means of a screw ; partly 

 for the purpose of placing each thread where it may be re- 

 quired, partly for enabling the observer to make a kind of 

 repetition in the observations with micrometers, which, with 

 the use of the clock-work that moves the refractor, is much 

 more practicable than it is in the common way of mounting. 

 In the same sense the eye-glass is separately moveable, in or- 

 der to make the two threads stand always equidistant from 

 the centre of the field of view, which makes them both equally 

 distinct. That part of the micrometer containing the threads, 

 supports, besides the necessary correction-screws, &c., two 

 verniers, in opposite positions, moving upon a graduated cir- 

 cle, made for the purpose of measuring the angles of position. 

 The verniers read off" to one minute. The micrometer may 

 be gently moved with respect to the position-circle, with the 

 hand or with a screw. The lines only are capable of being- 

 illuminated, so as to leave the rest of the field of view quite 

 dai'k. As the position-circle must remain unalterable with 

 respect to the position-axis, but the micrometer, together with 

 the apparatus for lighting the threads, must be capable of be- 

 ing turned, I was obliged to make a disposition of it different 

 from that which I had hitherto employed with micrometers 

 without a position-circle. The whole field may also be lighted. 

 This micrometer has four distinct eye-glasses. 



The refractor will receive moreover a lamp circle-microme- 

 ter with four eye-glasses ; a lamp net-micrometer with three 

 eye-glasses ; and finally, four ring-micrometers, two of which 

 contain double rings. 



As distinctness can only be properly obtained by the axis 

 of the object-glass and that of the eye-glass being exactly in 

 the same line, and a deviation in this respect being more in- 

 jurious in large object-glasses than in small ones, a particular 

 instrument will be added to the refractor, by which this de- 

 viation may be found and corrected. 



Fraunhofer. 



• This plate is inserted in the Atlron. Nach., and it may also be seen 

 in the 2d volume of the Memoirs of the Aslron. Society of London; but 

 it is too large for insertion in this work. 



VI. Oh 



