of the Motm's Distance from the Sun, ^c. 349 



A more correct use of the prisms is this : let them be capable 

 of being inclined to the rays which fall upon them ; then the 

 angle of deviation may be changed, by which means a number 

 of prisms will not be required. Tlie best method of applying 

 the prisms is to make use of two compound ones ; let AB, BC 

 (fig. 5) represent two prisms, the base of one joining the vertex 

 of the other; let DE be the ray incident upon AB, on leaving 

 the prism, its direction would be FGK, but after passing into 

 the other prism, is elevated, taking the dire^ction HL ; by varying 

 the angle ABC, or by inclining both the prisms differently to the 

 incident ray, the real angle of deviation LMD may =o, and from 

 « increase "to any required quantity. This latter method does 

 not require that the prisms be ground to any particular angle, 

 for any two prisms having very small vertical angles, equal or 

 "unequal, may be used. 



With respect to the circle GLM, if it be divided so far that 

 three or four minutes may be read off, it will be sufficiently ac- 

 curate, and in the arcs HK and QPG the degree may be divided 

 into six equal parts ; each very distinctly visible through a lens 

 of one inch focal distance, and greater accuracy than this is not 

 essential. 



Having only described the principle of the instrument, I pur- 

 pose in a future Number of this Magazine, to describe the me- 

 chanism minutely, and investigate the different formulae required; 

 also to ascertain the error in distance arising from any given error 

 in the computed distance, or altitudes. 



Description of the Eefieclifig Goniometer. 



This little instrument was first invented for the purpose of 

 measuring the angles of the prisms adapted to the sextant which 

 1 have just described; being in some respects more convenient, 

 and certaitdy capable of measuring an angle with a greater degree 

 of precision 'than most goniometers at present in use, it may not 

 perhaps be unacceptable to the unneralogist, 



ABC (fig. 3) represents a graduated brass circle ; in its cen- 

 tre is placed a plain mirror D at right angles to the plane of the 

 circle, capable of being turned round by a nut below the instru- 

 ment ; HK is an index carrying a vernier ; at a right angle to 

 this is fixed a brass pillar at a right angle, to which is attached a 

 small spring tul)e G, through which passes a rod of bras-, havmg 

 a plate of brase E, fixed at one extremity, and a milled nut at 

 the other F. • j • 



The application is obvious ; the mirror is adjusted, as m a 

 Hadley's sextant: the crvstal i^ attached to the plate E, by a 



httls 



