534 



pentine should be mixed, in order to produce this phenomenon, must be 

 understood to apply only to the particular specimens examined ; for 

 the rotatory power of oil of turpentine will be found to vary with 

 the specimen used, and also to some extent with the number of distil- 

 lations to which it has been subjected. To insure success, this ratio 

 must be determined by actual measurement for the specimens of oil of 

 turpentine examined. A certain amount of difficulty attends this mea- 

 surement, arising from the impossibility of obtaining a homogeneous 

 blue or violet ray of sufficient intensity for the purpose of the experi- 

 ment. Even when transmitted through a strong solution of sulphate 

 of copper supersaturated with ammonia, the beam will still contain 

 a considerable mixture of rays of different refrangibilities. The ro- 

 tations A h , F h , cannot therefore be measured with perfect accuracy. 

 It is possible, also, that, if the specimens of turpentine be not very 

 pure, there may be some chemical action between the two fluids 

 which will cause the rotation to vary. There may thus be a certain 

 amount of discrepancy between the observed and calculated results in 

 the case of the mixture. Thus in the present case the calculated rota- 

 tions are — 



For red light, . . . . - 1° 51', 

 For blue light, . . . . +2° 6'. 



Sir W. E. W. Wilde read the following unpublished Letter, written 

 by Lord Meath, July 14, 1690, respecting the Battle of the Boyne, and 

 addressed to an ancestor of Sir George Hodson, Bart., of Hcllybrook, 

 Bray, who had kindly permitted him to submit it to the Academy : — 



" Dublin, Tho s Court, July y e 5 th (90). 



" Madam, 



" Yesterday morning I entered this Tonn e ; after our 

 army & King James had a smart battle for 4 or 5 hou e rs on this side ye 

 Boyn e boath our whole armies being verey warmly engaged ; it hap- 

 pened ye first of July about 11: in y e morning where King William 

 forced y e passe on y e river Boyne called Old bridge 3 miles from Dro- 

 heda (in person) under y e en e mys Great gonnes mercey and ours ; with 

 email shott Like showers of Leden hale stones ; King William yewing 

 y e enemys Campe y e day before y e engagement was shott with an 8 

 pounder which tooke of his coat of his shoulders, and just drew blood from 

 his skinn; he called for a napkin and an other coat; and after y* was 

 settled upon him he stretched out his arme 3 times and sayd without 

 y e least passhion ; The en e my designed to prevent his fighting next day ; 

 But sertanly 1 11 be to-morrow amongst y e thickest of them. He was 

 just to his word ; for y e next day he fought through y e passe and with 

 losse on boath sides he tooke y e Battery of y e enemyes great gonnes and 

 killed all y e small shott men y* endeavored to ( ? ) it ; soone after 

 he charged y e enemy in y e Eeare broke there first lines, and our other 

 line being in y e front of theres ; wee drew up to Inclose y e enemy's 

 whole army ; but a damned (?) deepe bog being betweene we could not 



