and constructed hij Prq/vssor Aniici. 305 



upon a larger scale I shall bring it to a degree of perfection 

 that shall leave nothing to be desired. 



Note by Bai-on Zach. 



All those who know by theory and practice the reflecting 

 octant or sextant of Hadley, and who will compare it with 

 the reflecting sector of M. Amici*, will without doubt per- 

 ceive the innumerable advantages which this last instrument 

 has over the former. The octant of Hadley is founded 

 upon this simple principle ui catoptrics, — that if rays of light 

 diverging or converging are reflected by a plane mirror, they 

 diverge or converge after reflection towards another point on 

 the opposite side of this surface, and to the same distance as 

 the first point. It follows that if the rays of light coming from 

 any point of an object, be successively reflected by two plane 

 mirrors, a third plane perpendicular to the two mirrors, cross- 

 ing the point of emission, will also cross the two images suc- 

 cessively reflected. All the three points will be at an equal 

 distance from the common intersection of the points of the 

 real object, and of its image formed by the second reflection ; 

 they will make an angle double of that of the inclination of 

 the two mirrors. 



All the instruments of reflection ai'e constructed upon this 

 principle, and accordingly are furnished with two mirrors : a 

 thu'd is sometimes added, to make what is called the back ob- 

 servation : these three mirrors have from thence taken the names 

 of the great moveable mirror, and of small fore and back mir- 

 ror : the last two being fixed upon the instrument. 



Many scientific men have endeavoured to render perfect this 

 instrument, intended chiefly for the navy. Grant, Ewing, Dol- 

 lond, Magellan, Ramsden, T. Mayer, Borda, and others, have 

 modified and transformed it in various ways ; but the principle 

 has always continued the same, — the reflection of mirrors placed 

 at different distances, upon the plane of these instruments. 



The sector of M. Amici has no mirrors ; its reflections and 

 refi-actions are- made by two isosceles and rectangular glass 

 prisms, in the way in which the inventor of this instrument 

 has explained in his description. 



• M. Ainici has given to his instrument the name of Reflecting Sector : 

 the reflecting Octants or Sextants are instruments of an arc of 4.5° or (50°; 

 tliis of M. Amici is of more than 100°; thus it is ncitiier octant nor sex- 

 tant nor quadrant : it is a Sector. It is true, that in Astronomy an instru- 

 ment wliose arc contains but a few degrees, is denominated a Hector; hut 

 in Geometry, this classification exists not: besides, the two ivinds of instru- 

 ments will always be distinguisiied, the one being called a Zenith Sector, 

 tlie other a itcHecting Sector. 



N'ol. M. No. '291. Oct. 1822. Q 4 It 



