C m ] 



The telefcope mould be fixed by the inftrument- 

 maker fo as to command a full field of view when 

 the inftrument is placed at 90 if the inftrument be 

 an octant, or 120 if it be a fextant; becaufe the 

 index-glafs then ftands more oblique with refpect to 

 the incident and reflected rays, and confequently the 

 field of view of the telefcope, as far as it depends 

 upqn the index-glafs, will be more contracted than 

 in any other pofition of the index : but if there is a 

 fair field of view in this cafe, there necelfarily muft 

 be fo in every other pofition of the index. 



The two parallel wires will be very ufeful on many 

 occafions, as well in the fore as the back-obfervation. 

 In taking the altitude of the Sun, Moon, or ftar, 

 direct the fight towards the part of the horizon 

 underneath, or oppofite to the object, according as 

 you intend to obferve by the fore or back obfer- 

 vation, and hold the quadrant that the wires may 

 conftantly appear perpendicular to the horizon, and 

 move the index till you fee the object come down 

 towards the horizon in the fore-obfervation, or up to 

 it in the back-obfervation, and turn the inftrument in 

 order to bring the object between the wires j then 

 move the index till the Sun or Moon's limb, or the 

 flar touch the horizon. The nearer the object is 

 brought to an imaginary line in the middle between 

 the wires (it is indifferent what part of the line it is 

 brought to) and the truer the wires are kept perpen- 

 dicular to the horizon, the more exact will the ob- 

 fervation be. In the fore-obfervation, the object ap- 

 pears in its real pofition ; but in the back-obfervation, 

 the object being brought through the zenith to the 

 horizon, the real upper-limb will appear the loweft; 



and 



7 



