188 Astronomical and Nautical Collections. 



right with the ordinary on the left, there arrive at the same 

 time ordinary rays from the right hand and extraordinary from 

 the left, which, being polarised in a common direction, neces- 

 sarily influence each other, but produce no sensible fringes? 

 on account of the too great difference of their routes, or on 

 account of the too great distance of the point from the central 

 stripe, which for these two pencils is on the left hand : for 

 we have seen that, in white light, it is only possible to dis- 

 tinguish a very limited number of fringes, beginning from 

 the middle stripe, and that beyond the seventh or eighth order 

 the combination of the two pencils produces uniform light 

 only. The ordinary and extraordinary rays of each plate 

 are always found together at the same point of the enlight- 

 ened space ; but some of them form sensible fringes, by their 

 interference with rays of a contrary description coming from 

 the opposite plate ; while the others constitute a white light 

 only ; and from this distinction we are able to examine 

 them separately, and to determine the direction of their 

 polarisation. 



When two pencils which interfere are polarised exactly in 

 the same direction, the fringes which they form possess the 

 same character: but when the directions of their polarisation 

 form an acute angle with each other, the fainter fringes, 

 which they now produce, appear to be polarised at once in 

 both directions, since they disappear from the extraordinary 

 image when the principal section of the rhomboid is turned 

 either in the first or the second direction ; one of the pencils 

 being excluded in either case, so that the interference can no 

 longer take place, and the light must remain uniform. 



Having shown that these phenomena of interference con- 

 firm the general hypothesis, it remains to be proved that 

 they are inconsistent with the ingenious theory of moveable 

 polarisation, the fundamental principles of which it is neces- 

 sary to explain. 



Mr. BioT supposes, that when a polarised pencil passes 

 through a doubly refracting crystal, of which the principal 

 section is situated obliquely with respect to the primitive 

 plane of polarisation, the axes of the luminous particles, 

 which had been situated in this plane, undergo, at theirentrance 

 into the crystal, certain oscillations, which carry them alter- 



