C 301 ] 



LV. On a 7iexv Sextant recentlij invented and constructed by 



Professor Amici *. 

 ^■jr,- Modena, July 3, 1822. 



VViiEN in September 1820 you did me the honour to visit 

 me in my laboratory', amongst many instruments which I had 

 the pleasure to show you, your attention was particularly fixed 

 upon a combination of two glass prisms, by the motion of 

 which the angular distance of two distant objects might be mea- 

 sured. In the construction of this little model, my intention 

 was merely to give geographers and sailors a small commo- 

 dious instrument and one easily rectified for the measurement 

 of angles fi'om to 180 deg. within two or three minutes of the 

 truth : but the sketch of this instrument so much pleased you, 

 and vou perceived that such great advantages might be gained 

 b}' the use of it, that you encouraged me to pursue the idea, 

 anil to give greater perfection to it. Since that time I have, as 

 I promised jou, occupied myself about it ; and having com- 

 jileted this instrument, I have now the honour to transmit you 

 a description of it. 



If vv-e place a plane mirror before the object-glass of a tele- 

 scope, we easily perceive that two objects distant from one an- 

 other may be seen at the same time in the field of the tele- 

 scope, one by direct rays, the other by those that are reflected. 

 But if we desire. to see the same object directly, and at the 

 same time by reflection, it is well known that the thilig is im- 

 jiossible, because the rays are no longer reflected when they 

 are [)arallel to the reflecting })lane. 



It follows from this, that if at the object end of a telescope 

 a moveable jilane mirror be placed upon a graduated circle, it 

 cannot serve to measure angles, on account of the impossibility 

 of determining the coliimation ; that is to say, the beginning 

 of the division of this graduated arc. But if^ instead of a 

 mirror, we make use (as has been my practice) of an isosceles 

 and rectangular glass prism, besides the ol^taining a greater 

 (luantity of reflected light, we may also observe the coincidence 

 of two images ol'the same object, one produced by direct rays, 

 the other by reflected ones, and thus deternn'nc by this method 

 the zKuo point in the division of the instrument. 



Let, then, A BC, J'late IV. fig. 1, be a jn-ism placed before 

 theol)ject-glass E, so that its greatest face BA be in a line with 

 the axis of the telescope directed towards the distant object Q, 

 the jiarallel rays which come from one point of tlie object Q, 

 I'allir.g upon the side BC ol'the prism, will be returned by re- 

 fraction towards the plane BA, and alter having utidergone a 



* In :i Letter from the Professor to Raron Zach, roiitaincil in the Ccr- 

 rcupimd'in'-c .htroii'mii'/nr, vol. vi. pii^'c .">■'» J. 



l-Hal 



