Recent Advances in Electricity. 27 



the waves is modified. The influence of the particles will be 

 greatest when the natural period of vibration of the nucleus 

 (or one of its natural periods, if it have more than one) is 

 nearly or quite the same as the periodic time of the incident 

 waves, and this explains the intensely black absorption bands 

 and other singular phenomena exhibited in the spectra of certain 

 substances, such as fuchsine, phenomena known by the name of 

 " anomalous dispersion." In the case of an ordinary transparent 

 substance the theory tells us that this coincidence would occur 

 at a certain wave length, far beyond the hitherto known ultra 

 violet ; but beyond this point the influence of the nucleus will 

 diminish, and for wave lengths within a certain range the 

 influence on velocity of propagation will be practically nil, 

 which means that there will be no refraction. We are familiar 

 in many applications of science with actions which go on 

 increasing up to a certain point, and beyond this point diminish 

 again ; and it is claimed that this mav be true for refrangibility 

 as dependent on wave length. 



Among recent advances I must not omit to mention 

 the brilliant experiments exhibited by Tesla about five 

 years ago. They relate to the modification which takes 

 place in the ordinary discharges in vacuum tubes, when the 

 frequency of alternation is greatly increased. The fullest 

 account I have seen of his results is contained in the 

 "Journal of the Institute of Electrical Engineers,'' vol. 21, 

 No. 97, issued April, 1892 ; but several of his experiments 

 have become common property, and you may probably 

 have seen them at conversaziones. lie is rather indefinite 

 in his information as to the actual frequency of his alter- 

 nations, but he speaks of 10,000 per second and 20,000 per 

 second as low frequencies. One remarkable result concerns 

 the physiological effect. It is well known that alternating 

 currents give, as a rule, a severer shock than direct currents ; 

 but with the extreme rapidity of alternation employed by Tesla 

 no shock is felt at all. In many of the well-known Tesla ex- 

 periments the operator allows his own body to form part of the 



