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An Account of a Micrometer made of Rock Crystal. By G. Dollond, 

 F.R.S. Read January 25, 1821. [Phil. Trans. 1821, p. 101.] 



The author's object in this communication is to describe a more 

 simple application of rock crystal to the purposes of micrometrical 

 measurements in telescopes than any hitherto adopted. His im- 

 provement consists in the substitution of a sphere of rock crystal, in 

 place of the usual eye-glass, by which the trouble of angular cutting 

 is done away, it being only necessary to form the lens of a proper 

 diameter for the focal length required. 



Another advantage obtained by Mr. Dollond's micrometer is that 

 of being able to take the angle on each side zero without reversing 

 the eye tube ; and also of taking intermediate angles, by moving the 

 axis in which the sphere is placed ; and, thirdly, it possesses the pro- 

 perty of an eye tube not intended for micrometrical measurement, 

 for when the axis of the crystal is parallel to that of the object 

 glass, only one image is formed, and that perfectly distinct. After 

 adverting to some other advantages resulting from this improvement, 

 Mr. Dollond proceeds more particularly to describe the contrivance 

 by reference to an annexed drawing. 



The Bakerian Lecture. On the best kind of Steel and Form for a 

 Compass Needle. By Captain Henry Kater, F.R.S. Read Feb. 1, 

 1821. [Phil. Trans. 1821,^. 104.] 



On the return of the first expedition from the discovery of a 

 North-west Passage, the compasses were reported to have become 

 nearly useless, from the diminution of the directive force consequent 

 upon the near approach to the magnetic pole. The azimuth 

 compasses on that occasion being of the author's invention, he was 

 anxious that the second expedition should be furnished with instru- 

 ments combining the utmost power and sensibility ; and was con- 

 sequently led to the researches, the mode of conducting which, with 

 their results, form the subject of this lecture. 



In respect to the best material for the construction of compass 

 needles, Captain Kater found that clock springs made of sheer steel 

 were capable of receiving the greatest magnetic force, and that in 

 forming the needle it should be exposed as little as possible to heat, 

 by which its capability of receiving magnetism is diminished. 



The form best adapted for the needle is the pierced rhombus, of 

 about five inches long and two wide, and it should be tempered by 

 previous hardening at a red heat, and then softened from the middle 

 to about an inch from each extremity, by due exposure to heat, so 

 as to dissipate the blue colour. The polish of the needle appears to 

 have no effect upon its magnetism ; but in the same plate of steel, of 

 the size of a few square inches only, portions were found varying 

 considerably in their power of receiving magnetism, though not ap- 

 parently differing in other respects. 



The best mode of communicating magnetism to a needle appears, 



