4 Sketch of the Improvements in Science [Jan. 



Fiencli observations exhibit greater discrepancies than the English, 

 and that their apparatus was inferior. Finally, he infers, from the 

 goodness of the instruments, and the precautions taken, that the 

 error of Col. Mudge could not exceed half a second. Now all this 

 may be very true ; but the point can only be settled by repeating 

 the observations ; and till this is done it is obvious that well-founded 

 doubis may remain upon the subject. 



4. The comet which made so conspicuous a figure in the heavens 

 during a considerable part of 1811 could not possibly escape the 

 attention of philosophers. A very curious and accurate account of 

 all the particulars observed was drawn up by Dr. Herschell, and is 

 published in the Philosophical Transactions for 1812, p. 115. 



A second comet, observed at the end of IS 11 and beginning of 

 1812, is also described by Dr. Herschell in the Philosophical 

 Transactions for 1812, p. '22S>. 



A third comet was observed in July and August at Marseilles and 

 Paris. But it does not seem to have been visible in Great Britain. 

 Itj orbit was calculated by MM. Bourard and Nicolet, and it was 

 found not to lesemble any other before known. 



5. Mr. Dick has found by experience that the planet Venus may 

 be distinctly seen when only 3° from the sun, provided the direct 

 rays of the sun be intercepted from entering the telescope ; and he 

 thinks she may be seen even at the distance of l-i-°. 



6". Some valuable observations respecting the tides liave been 

 published by an anonymous writer in Nicholson's Journal, vol. 

 XXXV. pp. 145 and 217- 



7. Mr. Ez. Walker has determ.ined the latitude of Lynn, in 

 Norfolk, to be 52° 45' 24-4" N., and its longitude 1' 35-2" in 

 time E. from Greenwich. Phil. Magazine, vol. xli. p. 331. 



in. Optics. 



The discoveries in this important branch of science have been 

 curious and interesting. They originated with Malus ; and since 

 bis death have been prosecuted by Biot and Arrago in France, and 

 by Dr. Brewster in Scotland. 



If a rav of light fall upon one of the surfaces of a rhomboid of 

 Iceland crystal, and is transmitted through the opposite surface, it 

 is separated into two pencils, one of which proceeds in the direction 

 of the incident ray, while the other forms with it an angle of 

 6° IS'. The first of these pencils is said to experience the iisnal or 

 ordinary refraction, and the other the umisval or exlraordiuary 

 refraction. If the luminous object from which the ray proceeds be 

 looked at through the crystal, two images will be distinctly seen, 

 even wlien the rhomboid is turned round the axis of vision. If 

 another rhomboid of Iceland spar be placed behind the first, in a 

 similar position, the pencil refracted in the ordinary way by the first 

 will be so also by the second ; and the same thing holds with the 

 extraordinary relractcd pencil, none of the pencils being separated 

 into two, as before. But if the second rhomboid be slowly turned 



