806 On a nelo Hcxtant recently Invenled 



It results dii-ectly from this consti'uction, that the images re- 

 flected from the surface of these prisms lose not so much of 

 their light as if they were reflected fi'om mirrors. In Hadley's 

 octant, one image is always more weak than the other ; in the 

 sector of Amici, we may temper, modify and equalize the light 

 at plcasm-e, according to the motion given to the telescope N, 

 moveable upon the two pivots L and I (fig. 1 ) of the arm that 

 supports it, and according to the rotatory motion given to the 

 cap A with its coloured half-glass placed (as he has described) 

 before the object-glass. ' 



In the second place, in the sector of Amici, the instrument 

 has no parallax in the angles taken with objects very near to 

 the observer; because, from the construction of the instrument 

 the two prisms which give the two images are always quite 

 near to each other. If there be any parallax, it is so insigni- 

 ficant that it rarely merits consideration : nevertheless, if we 

 are fearful of it, we have only to determine the error of colli- 

 mation by the same objects of which we would take the 

 angles, and apply it to the angle indicated upon the limb of the 

 instrument. It is different in Hadley's octant, where the two 

 mirrors are placed at some distance from each other. The 

 octant marks upon the limb that angle which a ray from the 

 object reflected from the centre of the great mirror makes with 

 a ray from the other object from the centre of the small miri'or, 

 or, what comes to the same thing, from the observer's eye ; it 

 follows that when the first of these lines cuts the other pre- 

 cisely at the focus of the telescope, the octant then marks the 

 exact angle ; but if the intersection of these two lines falls any 

 where else, there will be a difference between the angle indi- 

 cated upon the limb, and that which the objects subtend, as 

 appears to the eye : we must then apply a correction to all those 

 angles made from very near objects. The treatises upon these 

 instruments teach the process for findmg these corrections. 

 This advantage in M. Amici's sector, of being exempt from 

 making any sensible parallax, will especially be appreciated by 

 navigators, who in making surveys of sea-coasts are often 

 obliged to make use of objects sometimes very near and some- 

 times at a great distance, to mark down the soundings on their 

 charts. 



In the third place, the crystd prisms of the sector of Amici 

 are not subject, like the mirrors, to flaws or cracks, and some- 

 tunes to the destruction of the silver. 



To avoid this inconvenience, mirrors of platina have been 

 proposed, and actually made : but this metal not being suscep- 

 tible of a fine polish, the old mode has been resumed. 



The glass mirrors are sometimes liable to be bent or 



waijied 



1 



