112 , IMPROVED HEl'tECTING CIRCLE. 



Huy^ens ap- which the ray of li^ht is divided ; thcrefci-e it will eive those 



plicable here, ,, . ^ , . 



likewise of the two pencils reiiected in the interior" of the crys- 

 tal. 



and in the Tf the two faces of the crystal be not parallel, we shall have 



case of sur- {^y (.{^g sanne law the directions of the two rays, into which the 



faces not pa- ^ 



rallel. generating ray is divided in entering by the first face : we shall 



have also, by this lavv'^ the direction of each of these rays at its 

 exit by the second face : next, the preceding construction will 

 give the directions, in the interior of the crystal, of the four 

 pencils reflected by this face : and, lastly, by the law of Huygens 

 we may diviace their directions at issuing out of the crystal by 

 the first face. Thus we shall have all the phenomena of the re- 

 flection of light by the surfaces of transparent crystals. Mr. 

 Malus first discovered these laws of the reflection of light, and 

 he has confirmed them by a great number of experiments. Their 

 agreement with the results of the principle of least action com- 

 pletes the demonstration of the position, that all these phaeno- 

 mena are owing to the action of attractive and repulsive fbrces*. 



V. 



Description "of a Rejlecling Circle, in which the Screens can le 

 readily shifted in taking altitudes : hy Mr. J. Allan, 

 Blewiti's Buildings, Fetter Lane.f 



SIR, 

 Improverl re- IT BEG leave to Inform you, that-on Thursday last I left at the 

 fleeting circle. J^ Society's house a marheraatical instrument, adapted for the 

 . use of mariners, which 1 wish to submit to the Society's atten- 

 tion. It is a reflecting circle, commonly called, La Borda's 

 circle, for the purpose of taking altitudes and distances at sea j 

 and which I have greatly improved lately, by fixing the shade 

 glasses different to what had heretofore been done, with some 

 other improvements as a reflecting circle. The late Dr.Mackay, 



« For the two papers of ^Mi*. ]^&las on this subject, see Journal, 

 vol. XXX pp. 95 and 161. 



f Trans, of the Soc of Arts, &c. ypl, XXIX* p. 106 The silver me- 

 dal jfnu tv/enrj guJneas were YOSed to Mr. Allan ior this improvement. 

 ' in a 



