222 



Mr. A. A. 0. Swinton. 



VTI. " On the Application of Harmonic Analysis to the Dynamical 

 Theory of the Tides. Part I. On Laplace's * Oscillations 

 of the First Species,' and on the Dynamics of Ocean 

 Currents." By S. S. Hough, M.A,, Fellow of St. John's 

 College and Isaac Newton Student in the University of 

 Cambridge. Communicated by Professor Gr. H. Darwin, 

 F.R.S. 



VIII. " On Boomerangs." By G. T. Walker, M.A., B.Sc, Fellow of 

 Trinity College, Cambridge. Communicated by Professor 

 J. J. Thomson, F.R.S. 



IX. " Cathode and Lenard Bays." By J. A. McClelland, M.A, 

 Junior Fellow R.U.I. Communicated by Professor J. J. 

 Thomson, F.R.S. 



X. " On the Structure and Affinities of Fossil Plants from the 

 Palaeozoic Rocks. — On Cheirostrobus, a new Type of Fossil 

 Cone from the Lower Carboniferous Strata (Calciferous 

 Sandstone Series)." By Dr. D. H. Scott, F.R.S. 



'< The Production of X Rays of Different Penetrative Values." 

 By A. A. C. Swinton. Communicated by Lord Kelvin, 

 F.R.S. Received March 24 —Read April 8, 1897. 



As is well known, if the X rays coming from an ordinary Crookes 

 tube of the Jackson focus type be observed with a fluorescent screen 

 daring the process of exhaustion, the penetrative value of the rays is 

 found to change as the exhaustion proceeds. 



First of all, at less than a certain degree of vacuum, no X rays are 

 produced. Next, as the vacuum is increased, X rays commence to 

 show themselves, but of a quality that will do little more than pene- 

 trate the backing of the screen. As the vacuum is further increased, 

 the rays become more penetrative, and show the shadow of the bones 

 in the hand. As exhaustion proceeds further, a point is reached 

 when the flesh of the hand seems to be almost completely transparent, 

 while the bones are almost entirely opaque. At higher vacua than 

 this, the bones becoming more and more transparent, the contrast 

 betw r een bones and flesh becomes less and less, till at length, at the 

 very highest vacuum at which the discharge will pass, the bones 

 scarcely show at all, owing to their having become nearly as trans- 

 parent as the flesh, while the whole hand throws but a very faint 

 shadow on the screen. 



Similarly, it is found that, at any given degree of vacuum, the 

 penetrative valuj of the X rays is increased by increasing the power 



