110 Mr Swan on Methods of ascertaining 



its centre in the same plane with G H ; and to F E are at- 

 tached the pieces K I, furnished with the screws I I, which 

 carry the glass C D. This glass has conical holes drilled in 

 it, for the reception of the points of the screws 1 1 ; and hav- 

 ing a free motion about the line joining their points, it accom- 

 modates itself to the surface of any prism that may be placed 

 between it and the glass AB. The prism, having its faces 

 slightly moistened, is thrust, like a wedge, between the glasses 

 until both are found to be in accurate contact with its surfaces. 

 This is ascertained to be the case by the fluid spreading uni- 

 formly between the prism and the glasses, and also by the 

 colours produced by the thin plate of intervening air or fluid. 

 When an accurate contact has been obtained, the prism will 

 retain its place firmly, provided too much fluid has not been 

 used ; but if any error be apprehended from its shifting, a 

 little soft wax applied at its base, to the glass A B, will prevent 

 it from altering its position. 



It will be sufficient to explain the use of this apparatus with 

 the theodolite, in order to shew its applicability to any other 

 angular instrument of the same kind ; and the manner of 

 using it with the sextant is so perfectly analogous to the me- 

 thod of observation already described, as to render any othep 

 description superfluous. 



In using the theodolite, it is first necessary to make the 

 line of coUimation of the telescope parallel to the horizontal 

 limb, which is then adjusted to the plane of the object to be 

 observed, either by raising or lowering the object, or by ad- 

 justing the plane of the limb by the parallel plate-screws. 

 The apparatus having been attached to the telescope, the 

 outer ring G H is inclined at a small angle to the line of col- 

 limation, and the telescope is turned aside until the image of 

 the object is seen by reflexion from the back of the glass A B. 

 The reflected image is then brought into coincidence with the 

 horizontal wire in the focus of the eye-piece, by turning the 

 collar K upon the telescope, and the collar is made fast by the 

 screw T. The telescope is now pointed 90° from the object, 

 and the ring G H is turned until the reflected image is again 

 visible. If it is now above or below the horizontal wire, it is 

 once more made to coincide with it, by altering the length of 



