of Secondary Electromagnetic Waves. 275 



observe even the second minimum. It will also be noticed 

 from the curve that the second minimum is at a distance from 

 the first slightly greater than the half wave-length ; so that 

 if the curve were continued further out we might very well 

 expect the minima to be shifted out the complete §-A ; as in 

 fact may be seen to be the case from the equation of the 

 curve. This in ordinary wave theory would be referred to the 

 necessity for the point to be at a proportionate distance before 

 a given surface can be considered as truly sending out a 

 secondary wave. Some experiments were made which cer- 

 tainly tended towards showing that this lengthening of the 

 distance between the first and second minimum exists ; but 

 the experiments were unsatisfactory, for the second minimum 

 is very slightly marked, and experimentally indeed seems to 

 be even less so than one would expect from the curve. How- 

 ever, it is most likely that the Hertzian stationary waves are 

 never so well marked as they would ordinarily be calculated ; 

 for it seems most probable that primary "vibrators" do not 

 send out strictly " monochromatic light," but send out a num- 

 ber of wave-lengths of nearly the same period — a "band- 

 spectrum," so to speak, the centre of which no doubt may be 

 by far the strongest, and correspond with the " period " as 

 would be calculated for the " vibrator ;" also that " secondary 

 vibrators " or resonators may in like manner be forced so as to 

 take up, in some degree depending on the discrepancy, any 

 member of a " band-spectrum " special to each resonator, its 

 "period" corresponding to the centre*. Thus the marking 

 of a node belonging to the central wave-length will probably 

 be weakened, owing to the presence of the other members of 

 the " band " whose nodes on the whole will occur elsewhere. 



An observation which has proved of use in the course of 

 these experiments on secondary waves was made during the 

 autumn, namely that glass absorbed Hertzian radiations com- 

 paratively rapidly ; in fact on account of this property it was 

 found quite impracticable to determine the velocity of " light " 

 in glass by the method j of interposing a sheet of the 

 substance in front of a reflector affording stationary waves, 

 because the front surface of the glass itself gave a well-marked 

 series of loops and nodes. A comparatively thin piece of 

 glass, say of one centimetre, will afford an observable reflexion. 

 This must be due chiefly to the reflexion from the second sur- 

 face being weakened by absorption, so that it is insufficient to 



* The author hopes shortly to publish an account of experiments made 

 during the autumn which have led to these conclusions, 

 t < Nature/ August 22, 1889. 



