102 Mr Swan on MetJiods of ascertaining 



Errata in the portion of the paper in Volume 35. 

 Page 347, line 19, /or *' 80 inches circuit" read " 180 inches circuit" 

 348, the letters c and d omitted at the poles of fig. 1. 

 348, last line, for " lightest" read " highest" 

 350, line 7, for " with my" read " with these" 



350, line 35, for " pole" read " pile" 



351, line '22, for " electrolice" read " electrolite." 



N.B. — Authors ought to be more correct in their M.S. in order to prevent lists 

 of errata. — Edit. 



On the Determination of the Index of Befraction by the Sextant y 

 and also by means of an Instrument depending on a new 

 Optical Method of ascertaining the Angles of Frisms. By 

 Mr William Swan, Teacher of Mathematics, Edinburgh. 

 (With a Plate.) Communicated by the Royal Scottish 

 Society of Arts.* 



The powers of transparent substances in refracting and dis- 

 persing the rays of light, present phenomena of the most inter- 

 esting description ; and the accurate determination of such 

 physical properties has always been regarded as an important 

 branch of experimental science. While these inquiries are 

 valuable in a scientific point of view, they are also of ob- 

 vious utility in relation to the useful arts, as affording the 

 means of constructing with accuracy many of the most important 

 optical instruments ; and, accordingly, the examination of re- 

 fractive and dispersive powers has occupied the attention, 

 not only of merely scientific observers, but also of the most 

 eminent practical opticians. Newton examined a considerable 

 number of substances ; and his sagacious conjecture of the in- 

 flammable nature of the diamond, and also of one of the in- 

 gredients of water, from then* great absolute refractive powers, 

 has been remarkably verified by modern discoveries. This 

 afibrds an example of the importance of the refractive index 

 as a physical character ; and it also shews the valuable results 

 that might be obtained by a more perfect knowledge of the 

 connexion between the chemical constitution of bodies and 

 their optical properties. 



* The paper of which this is an abstract, was read before the Society, 12th June 

 1843 ; and the Society's Gold Medal; value 15 Sovereigns, awarded 13th Novem^ 

 ber 1843. 



