On the Division of astronomical Instruments, 277 



c. Rectangular, a particular name given to the staurotide 



composed of two prisms which cross at right angles. 



d. ObUqueangied,^ particular name givtn to the staurotide 



composed ot" two prisms which cross at an angle oi: 60\ 



e. Sexradiated, a name given to the staurotide composed of 



three prisms which cross so as to represent the six 

 radii of a regular hexagon. 



f. Cruciform, a name given to the harmotome composed of 



two crystals wliich form a kind of cross. 

 g, Trighj [jhous y when the strice considered on three faces 



united around one and the same solid angle, are in 



three directions perpendicular to each other. Ex. Tri- 



glyphous sulphurated iron. 

 h, Geniculaicd, when it is conj posed of two prisms which 



rnite by one extremity forming a kind of knee. Ex. 



Geniculatcd oxidated titanium. 

 Tn the descriptions of the species wc shall meet with a small 

 number of denominations which we' have here omitted. But 

 their sigiiification will be immediately obvious, or will re- 

 solve itself into that of some of the foregoing denominations. 



[To be continued.] 



XLVI. On a Method of examining the Divisions of astro- 

 nomical Instruments, By the Rev, William Lax, 

 A,M,^ F.R.S., Lowndes's Piofessor of Astronomy in 

 the University of Camih'idge, In a Letter to the Rev. 

 Dr. Maskelyne, F.R.S. Astrunomtr Royal'^, 



y • St. Ibbs, August 27, 1808. 



Dear Sir, 1 am persuaded that you must feel, in common 

 with myself, how unpleasant it is to make use of an in- 

 strument in astronomical observations requiring extreme 

 accuracy, whose exactness you have no adequate means of 

 ascertaining, but are obliged to depend for it in a great 

 measure upon the abilities and integritv. of the artist. It 

 is in vain that we observe with so nmch niceiv, and read- 

 off with so much preciiiion, if we are still imeertain whe- 

 ther there may not be an error in the instrument itself of 

 much greater magnitude than those which we are en- 

 deavouring to, prevent ; and that our best instruments must 

 be liable to such errors, no person can possibly doubt, who 

 has paid due attention to the sources from whence they 

 may arise. T have estimated, as accurately as I could, the 

 ajTiount to which they niay accumulate in Bird's method 

 of dividing by continual bisections, and have satisfied my- 

 sdf that they arc much more considerable than is generally 



* J"rom the Philosophical Transactions for 1809, Part II. 



S 3 appre* 



