lOi Mr. J. Nixon 07i the Measurement of the 



rizontal line Y^e, it follows that when the bubble stands at 

 the intermediate distance, the line of collimation (EO) must 

 be in the direction of and coincident with the line of level E e. 



Proof-Tclcscopes. — These were the square telescope (mount- 

 ed a year ago with Foitin's level) and another, recently con- 

 structed by DoUond. It consists of a firm brass tube, 22 

 inches long and 1"5 inch in diameter, containing at one end a 

 stiff short tube, or drawer, fitted up with an excellent object- 

 glass, of 12'5 inches focus, and at the other end an inverting 

 eye-piece. At the focus of the object-glass is fixed a stop with a 

 vertical broad slip of pearl, which can be moved upwards or 

 downwards by opposite adjusting screws and nuts, until the 

 middle one of three beautifully fine lines drawn horizontally 

 across the slip about the middle of its length is brought to the 

 height required. The telescope rests within a pair of Ys 4*5 

 inches broad, cut out of a 2'5-inch plank of American birch, 

 which were glued (about a year ago) at a clear distance of 7 

 inches from each other to a (horizontal) l*.5-inch board (of 

 the same wood) 14< inches long and 6 inches broad. The pearl 

 slip being rendered vertical by moving the telescope about 

 within its Ys until it appeared parallel to the perpendicular 

 wire of the sector, the telescope was glued within the Ys. 

 After a lapse of a day or two, Lealand's level-tube, 14 inches 

 lonof, having a scale of about y^th of an inch for 2", was placed, 

 and secured by glue within the brass Ys, soldered, 10 inches 

 asunder, to the upper surface of the telescopic tube. 



The square telescope being adjusted to the proper focus by 

 pointing it on a clear day to a steeple at a distance of ten or 

 twelve miles, it was set up on the plank with Dollond's tele- 

 scope just beyond it, and the distance of the object-glass of 

 the latter from the pearl slip varied until the lines drawn across 

 it appeared, as viewed through the square telescope, at their 

 maximum distinctness. The short drawer containing the ob- 

 ject-glass was now secured to the tube at a slit in it, by driving 

 home a broad-headed screw working in the side of the drawer. 

 After the same method the object-glass of the sector was ad- 

 justed to the sidereal focus. 



The cross levels of the sector were successively adjusted to 

 their marks, when both extremities of the horizontal wire, as 

 measured by. the square telescope, had been found, from the 



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