372 Method of Increaa'mg the Divergency of 



approbation of them when Mr Witham's first work was finish* 

 ed ; and assuredly those of the second are in no degree infe- 

 rior. 



Lastly, Should any further remarks be offered on the subject, 

 I shall have an opportunity of representing matters injwhat 

 seems to me the true light in which they ought to be viewed ; 

 more especially if Mr Witham should not consider it necessary 

 to take up the subject. I shall also have the pleasure of present- 

 ing a historical view of the progress that has been made in the 

 examination of the recent coniferae, and, if possible^ of adding my 

 mite to the general store. 



In the mean time, although I have been obliged to speak for 

 myself, I am not disposed to claim the slightest merit for a 

 single fact of those published by Mr Witham. That gentleman, 

 having at great labour and expense collected numerous speci- 

 mens of fossil trees, employed a lapidary to slice them, and exa- 

 mined them with intense interest, brought the more characteris- 

 tic slices to me, explained the objects which he had in view, and 

 directed me to execute the drawings, and arrange the materials 

 which he had prepared for the work. Mr Nicol having also 

 made a collection of slices, gave me permission to select such as 

 I might find useful. Some of these were accordingly employed, 

 and, as I have said, of the manner in which they w^ere repre- 

 sented and described that gentleman repeatedly expressed his 

 approbation. During the progress of the second work, all com- 

 munication with Mr Nicol ceased. 



ON A METHOD OF SO FAR INCREASING THE DIVERGENCY OF THE 

 TWO RAYS IN CALCAREOUS SPAR, THAT ONLY ONE IMAGE 

 MAY BE SEEN AT A TIME.* 



Under this title, Mr Nicol some years ago described in the 

 Edinburgh Philosophical Journal, (No. XI. p. 83.) an instru- 

 ment which, at first sight, does not seem to possess any superi- 

 ority over a good achromatised prism of calcareous spar, but 

 which, however, I am convinced from my own experience, does 

 possess important advantages, inasmuch as it throws the ima- 



• From Poggendorf*8 Annalen der Physik und Chemie, vol. xxxix. p. 182. 



