FRESNEL. ON POLARIZED LIGHT. 45 



With the glass alone, I did not perceive more rings through 

 the oil of turpentine than before the interposition of that liquid ; 

 but on placing a rhomboid of carbonate of lime in the interior 

 of the telescope, so as to produce two separate images, I per- 

 ceived in each of them a considerable increase in the number of 

 rings : they vi^ere perceptible even when the film of air was of 

 that thickness at which I had previously never been able to dis- 

 cover them *. Now, one can only explain the appearance of 

 these new rings by supposing a diminution in the interval of 

 the two systems of waves, which combine to produce them ; or, 

 what comes to the same thing, by supposing that the one part 

 of the system of waves reflected by the first surface of the film 

 of air, traversed the tube a little more slowly than a part of 

 those reflected from the second surface. Thus it must be ad- 

 mitted that the oil of turpentine retards, like crystals, the passage 

 of light in two different degrees. As the rays reflected by the 

 first and second surface of the film of air must equally suffer 

 double refraction in passing through the liquid, the new rings 

 can only be formed by one half, at most, of the light which 

 reaches the eye ; so that they ought to be much more feeble 

 than the others. 



It may be objected to the deductions which I have just made 

 from this experiment, that the circumstances giving rise to the 

 new rings being precisely those which cause the colours in the 

 oil of turpentine, the apparent augmentation of the number of 

 rings may possibly be due to the simplification of the light. 

 But in the first place, I reply, that these colours were exceed- 

 ingly feeble in consequence of the great length of the tube, and 

 that even, in certain positions of the rhomboid of calcareous spar, 

 they became insensible ; the two images then appearing to have 

 no other colour than that peculiar to the liquid. It will be seen 

 besides, that several other phaenomena confirm the hypothesis of 

 double refraction in the oil of turpentine. 



Having carried the same tube into a dark room, I du'ected it 



* M. Arago made, a long time ago, a perfectly similar experiment upon 

 plates of rock-crystal, cut at right angles to the axis. The same phsenomenon 

 can be produced with laminae of rock-crystal or sulphate of lime, cut parallel 

 to the axis, and of but slight thickness. When they are only 1 or 2 millimetres 

 thick, the new rings are perfectly separated from those which surround the 

 point of contact, and establish with certainty the double refraction of the cry- 

 stal. This property of crystalline laminre may be equally well applied as a 

 measure of their doubly refractive jiowers, their thickness, and of the curva- 

 tures of the object-glasses of the telescope. 



