308 SURVEY OP THE COAST 



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direction, and presents the divisions without any false shade, 

 and very distinct. In any other position of the light or of 

 the reflection, the strokes, which always present cavities, 

 though they appear filled, will be viewed on the side, as one 

 side of this cavity will always be in the light and the other 

 in the shade ; and the influence of this upon the accuracy of 

 the reading is much greater than might be imagined, exceed- 

 ing the limits within which it is possible to read with these 

 microscopes. 



For this same reason, night observations have not the same 

 degree of accuracy on the horizontal limb of a theodolite as 

 those made during the day ; because in this case it is impos- 

 sible to make the light fall in the proper direction, as no 

 light can be placed in the centre, and outside, in the direction 

 of the radius, it would occupy the place belonging to the ob- 

 server. Night azimuths lose much on that account, and sel- 

 dom give the satisfaction expected. 



On the Sig?ials and the System of Wires in the Telescope. 



An object closely connected with the accuracy of the ob- 

 servation of terrestrial angles, is the choice of proper and dis- 

 tinct signals, and the adaptation of the system of wires to 

 them. 



In dip first place, 1 must observe that objects seen from a 

 great distance are visible rather in proportion to their differ- 

 ence in light and colour from the surrounding objects or the 

 back ground on which they are projected, than in proportion 

 to thfir size, which actually contributes very little towards 

 the effecting of the vision, and is always detrimental to its 

 dihiinctness. A small object seen by the shaded side, if pro- 

 jected upon a clear sky, will be visible at a great distance, 

 and will be much more distinct, if it reflect the rays of the 



