VOL. LXXVI»] PHILOSOPHICAL TRANSACTIONS. 15Q 



completed. As I may perhaps find an opportunity hereafter to publish these ob- 

 servations at full length, I shall now only mention such circumstances, relating 

 to the instrument and apparatus with which they were made, as will be necessary to 

 show what degree of accuracy may be expected in the determination of the places 

 of these nebulae and clusters of stars; and also to serve any astronomer, who 

 wishes to review them, to form a judgment what instrument will suffice for this 

 purpose. 



The telescope I have used, as has been observed on a former occasion, is a 

 Newtonian reflector of 20-feet focal length, and 18-r^ inches aperture. The 

 sweeping power has been 157, except where another is expressly mentioned. 

 The field of view 1 5' 4". My eye-glass is mounted on that side of an octagon 

 tube, which, in the horizontal position of the instrument, makes an angle of 

 45° with the vertical ; having found, by experience, that this position, resem- 

 bling the situation of a reading desk, is preferable to the perpendicular one com- 

 monly used in the Newtonian construction. In the present improved state of 

 the apparatus this telescope will, in general, give the relative place of an object 

 by a single observation true to within 14 or 2 minutes of polar distance, and 4 

 or 6 seconds of time in right ascension. But when there is an opportunity of 

 repeating the observation, it will hardly differ a single minute in the former, and 

 seldom so much as 3 or 4^ in the latter. My apparatus however has not been 

 equally perfect from the beginning; for, being from time to time adapted to the 

 different views I had in sweeping, it could only arrive to its present degree of per- 

 fection by many experiments and gradual improvements. 



To begin a short history of this 20-feet telescope. In the month of October 

 of the already mentioned year I began to use it, being then mounted on its pre- 

 sent stand, but with a lateral motion under the point of support of the great 

 speculum, by which its direction could be changed about 1 5 degrees. It had 

 also a kind of moveable gallery in front, about 9 feet long, which permitted me 

 to follow a celestial object near 15 degrees more; by which means I obtained a 

 range of 30 degrees without moving the stand. The Newtonian form has the 

 capital advantage of rendering observations equally commodious in all altitudes; 

 I had therefore placed the instrument in the meridian, that I might view the 

 stars in their most favourable situation. 



When I had seen most of the objects I wished to examine, J proceeded to the 

 work of a general review of the heavens. The first method that occurred was, 

 to suffer the telescope to hang freely in the centre; then, walking backwards and 

 forwards on the moveable gallery, I drew the instrument from that position by a 

 handle fastened to a place near the eye-glass, so as to make it ^bllow me, and 

 perform a kin^l of very slow oscillations of 12 or 14 degrees in breadth, each 

 taking up generally from 4 to 5 minutes of time. At the end of each oscillation 



