1821.] Philosophical Transactions for 1821, Part I. 229 



The standard used in the Trigonometrical Survey, being thus 

 unexpectedly found, Capt. Kater observes, to differ so consider- 

 ably from every other standard of authority, the Commissioners 

 of Weights and Measures proposed in their second report, that 

 Bird's parliamentary standard of 1760, should be considered as 

 the foundation of all legal weights and measures. 



It is remarked by Capt. Kater, " that the standard thus 

 selected differs so little, if at all, from that of Sir G. Shuckburgh, 

 that they may, for every purpose, be considered as perfectly 

 identical ; and this agreement is particularly convenient, because 

 the length of the metre having been determined by comparisons 

 with Sir G. Shuckburgh's scale, and a fac simile of it made by 

 Mr. Troughton, for Prof. Pictet, all measures on the Continent 

 are converted into English measures by reference to Sir George 

 Shuckburgh's standard." 



VIII. An Account of the Urinary Organs and Urine of two 

 Species of the Genus Rana. By John Davy, MD. FRS. 



An account of the contents of this paper has been given in the 

 Annals. 



IX. An Account of a Micrometer made of Rock Crystal. By 

 G. Dollond, FRS. 



This paper oannot be rendered completely intelligible without 

 reference to the drawing which accompanies it. According to Mr. 

 Dollond the advantages to be derived from his improvement are 

 the following : that in making a sphere or lens from a piece of 

 rock crystal, and adapting it to a telescope in the place of the 

 usual eye glass ; and from its natural double refracting property, 

 rendering it useful as a micrometer. 



The advantages of thus applying the crystal are, in the first 

 place, the very great saving of the time required to find the pro- 

 per angle for cutting the crystal, also of cutting the crystals to 

 their proper angles, and working their surfaces with sufficient 

 accuracy to render them useful as micrometers in the manner 

 that is recommended by M. Arago, Dr. Wollaston, and 

 others. 



Upon the plan which is now submitted, it is only necessary to 

 select a piece of perfect crystal ; and without any knowledge of 

 the angle that will give the greatest double refraction, to form 

 the sphere of a proper diameter for the focal length required. 



The second advantage is derived from being able to take the 

 angle on each side zero, without reversing the eye tube; also of 

 taking intermediate angles between zero and the greatest sepa- 

 ration of the images, without exchanging any part of the eye 

 tube, it being only required to move the axis in which the 

 sphere is placed. 



Thirdly, it possesses the property of an eye tube or lens that 

 is not intended for inicrometrical measurements ; for when the 

 axis of the crystal is parallel to the axis of the object glass of the 



