546 l)r. Tyndall on the Pi'ogress of the Physical Sciences : 



nipulation. The divergence spoken of is in reality exceedingly 

 small. In order to observe it, M. Foucaiilt has made use of a 

 square aperture furnished with a number of vertical bars of fine 

 platinum wire ; eleven of these fitted in the space of one milli- 

 metre, and between each two there was a small space through 

 which the light entered. The image given by this was a small 

 field furrowed with alternate l^lack and white stripes. The light 

 after entering through this aperture fell upon a lens by which it 

 was converged, but before it came to a focus on the opposite side 

 it fell upon the rotating mirror ; it was thence cast upon a con- 

 cave mirror placed about six feet distant, which i-eflected it back 

 again. By a peculiar artifice M. Foucault was enabled to com- 

 pare with great nicety the divergence of the black and white 

 stripes in the image from the platinum wires and their inter- 

 vening spaces. " I have already proved," says M. Foucault, " by 

 two successive operations, that the deviation of the image after the 

 journey of the light through air is less than after its journey through 

 water. I have also made another confirmatory experiment, 

 which consists in observing an image formed partly by light 

 which has passed through air, and partly by light which has 

 passed through water. For small velocities, the stripes of this 

 mixed image were apparently continuations of each other. But 

 by the acceleration of the motion the image is transported, and the 

 stripes are broken at the point of junction of the air image with 

 the water image. The stripes of the latter take the advance in the 

 sense of the general deviation. Further, on taking into account 

 the length of tvater and of air traversed, the deviations are found 

 to be proportional to the indices of refraction. These results indi- 

 cate a velocity of the light which is less in water than in air, and, 

 according to the views of M. Arago, fully establish the theory of 

 undulation." 



The following interesting article on the same subject is from 

 MM. Fizeau and Breguet. 



We have realized with great exactitude the experiment de- 

 scribed in om- note presented to the Academy during its session 

 of the 6th of May last ; an experiment which we felt called upon 

 to make, although M. Foucault in the same session had read an 

 extended paper upon this subject, in which he announces that he 

 has already obtained decisive results. 



We have thought that, for the solution of a capital question 

 like the present, the proofs could not be too much multiplied, 

 and that experiments made under different circumstances could 

 not but contribute to render our knowledge of an important fact 

 more certain. 



We have applied ourselves to the solvition of the question as 

 proposed by M. Arago in 1848 ; that is to say, How can the 



