722 Notices respecting New Books. 



hence I argue that we are justified in ignoring the infini- 

 tesimal intervals of* time which a representative point may 

 spend on them, and in predicating equipartition of energy &c. 

 to our unknown gas. This argument, I claim, assumes 

 nothing but the laws of dynamics. 



Mr. Burbury's objection to my proof is, in effect, that I 

 assume all combinations of the 3 N velocities with the 3 N 

 coordinates to be equally probable. Of course in constructing 

 my generalized space, I regard the 6 N quantities as abso- 

 lutely independent variables, but this is a different matter : 

 I may make any possible construction I please, just as if I 

 w^as proving a proposition of Euclid. If, now, all combina- 

 tions of the 3 N velocities and 3 N coordinates are not equally 

 probable — if, that is, the motion of the gas, in accordance 

 with the laws of dynamics, tends to set up correlation between 

 the 3 N velocities and the 3 N coordinates — then the fluid 

 in my generalized space will tend to congregate about the 

 regions in the generalized space at which this correlation 

 obtains. This, I prove by Liouville's Theorem, does not 

 occur. I therefore p7'ove the absence of correlation between 

 the velocities and coordinates : I cannot see that I assume it. 



Yours faithfully, 



J. H. Jeans. 



XCI. Notices respecting New Books. 



Manual of Advanced Ojotics. By C. Kibokg Manx, Assistant Pro- 

 fessor of Physics in the University of Chicago. Chicago : Scott, 

 Foresman and Co. 1902. Pp. 196. 



T^HIS excellent little manual of a laboratory course in advanced 

 -"- optics fills a distinct gap in scientific literature, and deserves a 

 warm recognition. It is just the sort of book which many advanced 

 students must have been longing for. AVhile giving full practical 

 instructions for the setting up and adjustments of the various forms 

 of apparatus used, the author prefaces each experiment by an 

 exposition of its theory. The experiments are well chosen, and 

 the necessary apparatus is not expensive. The subjects dealt with 

 are : limit of resolution, the double slit, the Fresnel mirrors and 

 bi-prism, the Michelson Interferometer, the visibility curves, the 

 prism spectrometer, total reflexion, the diffraction grating, the 

 concave grating, polarized light, rotation of the plane of polariza- 

 tion, elliptically polarized light, reflexion of polarized light from 

 homogeneous transparent substances, metallic reflexion, the spec- 

 trophotometer. At the end of the book are two interesting 

 chapters on " the development of optical theory " and " the trend 

 of modern optics." 



