544- Dr. Tyiulall on the Progress uf the Physical Sciences : 



still amongst the desiderata of the subject. A few mechanical 

 problems also occur, chiefly statical, the solutions of which are 

 strongly marked with the geometrical taste and elegance of the 

 wm-iter ; but though they would be admirably suited to an ele- 

 mentary work, they are not of a kind to require further specifi- ■ 

 cation here. 



LXXVII. Reports on the Progress of the Physical Sciences. 

 By John Tyndall, Ph.D., Marburg. 



On the Velocity of Light. — Experimental Proof of the Theory of 



\v\ •;\5'>:)0 ^o.■vs^^>•^ ^IMdulation. ''- 



]. HMJiocte iji^n/rate pour mesurer la vitesse de la lumiere dans I'air et 

 tes milievx transparents, by M. L. Foucault. Comptes Rendus, 

 May 6, 1850. 

 i2. Sur la Vitesse comparative de la lumiere dans Pair et dans I'eau, by 

 MM. II. Fizeau and L. Bieguet. Ibid. June 17, 1850. 



THE state of Arago's sight has recently induced him to resign 

 the cariying out of his proposition of submitting the two 

 rival theories of light to experimental decision to physicists gifted 

 with younger eyes. It is well known, that, according to the 

 theoiy of Newton — the so-called emission theory — the velocity 

 of light in passing from a rarer medium into a denser is increased. 

 For example, the index of refraction in passing from air to water 



4 ... 



is ^ ; according to the emission theory, the velocity of light m 

 o 



air is to its velocity in water as 3 is to 4. Opposed to this 



stands the theory of undulation, proposed by Huyghens, and 



supported by Eu'ler, Young and Fresnel. According to this 



theory, the velocity of light in passing from a rarer to a denser 



medium is diminished ; in the case of air and water, for instance, 



the above ratio is reversed ; the velocity of light in air is to its 



velocity in water, in the ratio of 4 : 3. The genius of Fresnel 



has won for the latter theory almost universal recognition ; a 



direct proof was however wanting, and this urged Arago* to the 



hardy thought of submitting the question to an experimental test. 



The rotating mirror of Mr. Wheatstone was proposed as the 



instrumental agent for can-ying out this idea. If we conceive a 



ray of light to enter a dark room through a hole in a window- 



* It ought to be mentioned, that the same subject had occupied the 

 attention of Sir John Ilerschel and Mr. Wheatstone some yeai's before it 

 was mooted by Arago ; and a proposition was actually made by the former 

 to send a bar of light through a tube of ;^ater or alcohol a mile in length, 

 and thus determine the influence of this medium. This idea, though not 

 so jiractical, is the same in principle as that of Arago. The latter, however, 

 was not aware that any such proposition had ever been made. . 



