124 Method nf adjust ing Hridlcij's Sextant. 



will give, if not exactness, at least a demonstrable deviation 

 within 15 seconds, I shall feci myself happy if what I com- 

 municate prove of service to others in its present state, or 

 may give rise to any new ideas of improvement in the in- 

 strument itself. 



It is a known principle in optics, that the angle of re- 

 flection is equal to the angle of incidence : hence the angle 

 formed by B and C in fig. 1. (Plate III.) being greater than 

 a right angle B receiving a ray and transmitting it to C, 

 the consequence from the above principles is, AD becomes 

 greater than A E ; AE being equal to the distance of the 

 reflecting points in B and C in the second figure, B and C 

 forming a right angle, E and D coincide in the third figure ; 

 B and C forming an angle less than a right angle, AD be- 

 comes less than AE. 



Let us now suppose the eve placed behind G in fig. 4, 

 so as to obser\-e or bring the point G upon A, and behind H 

 in ihc line BH let the flame of a candle be placed; it is 

 evident that the ray HB from the flame will fall upon B, 

 be reflected to A, and be again reflected by A in the direc- 

 tion AG, so as to be distinctly seen bv the eye behind G. 



Unscrew the lever of the back observation glass, and, 

 turnins; it round, adjust it like the fore observation glass : 

 in this attempt it will be found necessary to remove the 

 sight-vane to a new situation, which mav be done by glue- 

 ing a small bed of wood on the side at Z to hold it *. 



Having 



• It may be useful here to remark, that in upwards of looo altitudes 

 of the sun, taken with my instrument with the back observation glass, 

 turned as here described, and the sight-vanc in a bit of wood fastened to 

 the side, and all the opening of the back observation glass covered by 

 pasting p:i^ier over its surface, except a strip exactly as bro^d as the open- 

 ing, and at right angles to it, 1 have found, upon taking any even number 

 of double altitudes of the sun, with their correspondent times, that when 

 the avcra2:e was found it a/ii-'JM accordtd in time and riltitude with the 

 middle ^)Uce; whereas, when I have tried tlic furc observation glass in 

 the same manner, it wiis frequently subject to deviation, and vcrv seldom 

 would bear apportioning, when both occupied the middle of tliC column. 

 Let an example, taken yesterdav, testify to what I assert. 

 ■iSo4j rtUruary 13 X 14-15 - 33°5i'i5'' 



