Astronomy— Optics. 1 77 



8. Mr Pond's Method of Determining the Direction of the Meridian. 

 This method consists in placing two very distinct and well defined objects 

 nearly at equal distances on each side of the north meridian line, and at a 

 distance from each other nearly equal to twice the greatest elongation of 

 the pole star. These objects are placed by means of a telescope, fixed 

 like a transit on a horizontal axis, and pointed first to the pole star, at its 

 greatest elongation, and then at its reflected image. In thus passing from 

 the star to its image, the central wire, which must necessarily describe a 

 strictly vertical circle, will pass over some terrestrial object, which, if well 

 defined, will serve for the mark ; but if not, a mark must be set up so as 

 to be bisected by the wire. The same being done on the other side of 

 the north meridian, the distances of the marks from the central wire 

 must be accurately measured with a micrometer. The horizontal angles 

 between the marks being measured by a theodolite, a meridian mark is to 

 be erected exactly between them. 



9. Remarkable Effect on the Cambridge Transit Instrument. In ad- 

 justing the transit instrument recently put up in the New Observatory at 

 Cambridge, Mr Woodhouse observed, that the line of collimation deviated 

 occasionally to the east or west of the meridian mark without any visible 

 cause. He at last found that this effect arose from the approach of the 

 assistant's body to the lateral braces used to steady the instrument in an in- 

 variable position at right angles to its axis. The expansion of the brace 

 nearest him thrust the axis of the telescope aside. Mr Woodhouse prov- 

 ed that this was the cause of the deviation, by producing the same effect 

 with hot cloths wrapped round the alternate braces, and he, therefore, pro- 

 vided a proper apparatus to protect the braces from the solar heat, during 

 the approach of the sun to the meridian. 



OPTICS. 



10. Committee for the improvement of Glass for Optical purposes. We 

 understand that the Royal Society of London, and the Board of Longitude, 

 are taking effectual measures for the improvement of glass for optical pur- 

 poses. A regular Glass-house has been built for this purpose, and a series 

 of experiments will be immediately begun, under the direction of a com- 

 mittee of members of the Royal Society, from whose talents and dili- 

 gence we anticipate the happiest results. 



11. Traces of Polarized Light in Huloa. M. Arago is said to have 

 remarked unequivocal traces of polarization by refraction in the light 

 of a halo, seen round the sun towards 11 o'clock in the morning; and 

 he concludes, that halos cannot, therefore, be formed by reflection. 



12. Telescopes ivithoul Irradiation. M. Arago has announced, that he 

 h:is constructed telescopes in which there is no irradiation. The diameter 

 of one of the satellites of Jupiter, and that of its shadow on the planet, 



VOL. IV. NO. I. JAN. 1826. M 



