[ 524 ] 



LXXIV. On an alleged Demonstration of Tresnel relative to 

 the Wave-suiface in the TJieory of Double Refraction, By 

 John Tovey, Esq, 



To the Editors of the Philosophical Magazine and Journal, 



Gentlemen, 

 I AM glad to find from Mr. Lubbock's paper in your last 

 -*■ Number, p. 417, that he is devoting a portion of his ma- 

 thematical talents to physical optics. His comparison of 

 Fresnel's ideas with those which have been since developed 

 by M. Cauchy and others, is, in the present state of the sci- 

 ence, of great importance. I hope he will continue these 

 investigations; and I beg that you will allow me, through the 

 medium of your Journal, to solicit his consideration of that 

 part of Fresnel's theory in which it is supposed to be proved 

 that when the axes of elasticity are taken for the coordinate 

 axes, the differential equations may be reduced to the form 



~j = m *.{ <fr (r) + * (r) Ax*\ A % 



Mr. Lubbock takes this point for granted, referring for a 

 proof of it to the remarks of Fresnel in the Mem, de Vlnstitut. 

 To these remarks I have no access ; but I conceive that they 

 can be no more than equivalent to the demonstration of C.J. 

 in your last volume, p. 24, which only proves, that if one mo- 

 lecule alone of the system be displaced, it is always possible so 

 to take the axes of the coordinates that the differential equa- 

 tions may be reduced to the form 



^| = Af.«j{»(r)+*(r)4i'}. 



Now this equation is inapplicable when the system is in a 

 state of undulation; and, therefore, unless it can be proved 

 that it may then be changed into the form made use of by 

 Mr. Lubbock, Fresnel's demonstration must be abandoned. 



I am, Gentlemen, yours, &c. 

 Littlemoor, near Clitheroe, John Tovey. 

 Nov. 6, 1837. 



LXXV. A Report of the Progress of Vegetable Physiology 

 during the Year 18.36. By J. Meyen, Professor of Botany 

 in the University of Berlin,* 



[Continued from p. 446.] 

 On the Structure and Function of Spiral Tubes, 

 r I^HAT the spiral tubes in vegetables serve to conduct the 

 nutritive sap has once more been remarked by Link-j-, in 



* From Wiegmann's Archiv fur Naturgcschichtc,\%3'] ', Part 3. Trans- 

 lated by Mr. Wm. Francis. f Phihs. Bot., p. 189. 



