Astronomical and JVautical Collections, 40*^ 



provided that they had always the same directions in the two 

 interfering portions of the unduhitions. According to this 

 new hypothesis, common light must be a combination, or 

 rather a rapid succession of an infinite number of undula- 

 tions, polarised in all manner of directions : and the act of 

 polarisation must be considered, not as creating transverse 

 motions, which already exist in common light, but in decom- 

 posing them according to two invariable rectangular planes, 

 and in separating from each other the systems of undulations 

 polarised in these two directions, either by altering their 

 general direction, or simply by means of the difference of 

 their velocities. 



Experiments, as well as the principle of interference, have 

 taught us, that when a pencil of polarised light is divided 

 into two systems of undulations of equal intensities, polarised 

 in rectangular directions, and separated by the interval of a 

 quarter of an undulation, it exhibits upon the reunion of the 

 two systems of undulations, the appearance of complete 

 depolarisation, that is to say, that the whole light, when 

 analysed with a rhomboid of calcarious spar, gives always 

 images of equal intensities, in whatever direction we may 

 turn the principal section. The light, thus modified, resem- 

 bles in this respect direct light ; but it differs from it by 

 some very curious optical properties which are the principal 

 subject of a memoir that I communicated to the Academy of 

 Sciences, the 24th of November, 1817. 



[To be continued in our next Number.] 



ii. Remarks on Mr. Henderson's Imjirovement on Dr. 

 Young's method of computijig the Longitude from the 

 observed occultation of a fixed Star by the Moon: with 

 Mr. Henderson's Answer, 

 ** When neither of the altitudes has been observed, the 

 computation of that of the moon is liable to considerable 

 uncertainty, as depending upon the supposed longitude by 

 account." This is very true; and, in fact, it ought never 

 to be attempted. Neither, in strictness, ought it to be ob- 

 served, unless with a very good sextant, and under very 

 favourable circumstances with respect to the horizon ; and, 

 if observed, the star's altitude ought also to be observed; 



