Mr. Nixon's Theory of the Telescopic Level. 



When any point of an object viewed through a refracting 

 telescope is intercepted or apparently covered by the inter- 

 section of its cross-wires, that point of the object is in the di- 

 rection of a straight line, termed the line of sight or collimation, 

 passing through the intersecting point of the wires and the 

 centre of refractions, or thickest part of the object-glass. 

 When the telescope and wires are properly adjusted, the eye 

 may be gradually withdrawn laterally from its original posi- 

 tion in front of the eye-glass, without perceiving the least pa- 

 rallax, or deviation in the direction of the wires. Were wires, 

 substituted for the object-glass, made to cross each other at the 

 precise point previously occupied by its centre of refractions, 

 we should find, on withdrawing the eye-tube, that a straight 

 line passing through the intersection of the original and that 

 of the substituted cross-wires (together forming what are 

 called plain sights), would be in the direction of the same 

 point of the object observed. The centre of the object-glass 

 is known to coincide with that of its cell, and is consequently 

 situated in, or very nearly in, the direction of the axis of the 

 cylindrical tube, when the line of collimation, during a revo- 

 lution of the glass within its cell, does not deviate from the 

 object to which it was previously pointed. All the rays of 

 light from a fixed star which enter a telescope are sensibly 

 parallel to each other, but converge, after having passed 

 through the object-glass, to a point (?) within the tube at a di- 

 stance from the object-glass, called the sideral focus. Hence 

 rays of light passing out of a telescope from the point of in- 

 tersection of the wires, placed at the sideral focus, are inclined 

 to each other, and diverge until they reach the object-glass, 

 in which they suffer refraction, and emerge perfectly parallel 

 to each other*. When two telescopes, properly adjusted with 

 their cross-wires at the sideral focus, are placed nearly in a 

 line, but in opposite directions, the wires of the further tele- 

 scope, on looking through the nearer telescope, will appear 

 perfectly distinct, whether the two object-glasses are in im- 

 mediate contact or at a distance from each other. In either 

 case, if the intersecting point of the wires of the nearer tele- 

 scope is made to coincide with or intercept the view of the 

 intersecting point of the wires of the further telescope, the 

 lines of collimation of both telescopes will be strictly parallel 

 to each otherf. They cannot however be in one line (or di- 

 rection), unless ( which we cannot ascertain (?) ) the centres 

 of refraction of both object-glasses and the intersecting points of 

 both cross-wires are all in the same straight line. Hence we 



* Prof. Gauss of Gcettingen. f Prof. Bessel of Kcenigsberg. 



JV.S. Vol. 9. No. 54. June 1831. 3 I may 



