Dr. Oliver Lodge on Opacity. 399 



or from the transmission of lio-ht through conductors. So that 

 the red of the sunset sky and the green of gold-leaf may not be 

 after all very different ; nor is the arrival-curve of a telegraph 

 signal a wholly distinct phenomenon. 



There is a third kind of opacity, that of lampblack, where 

 the molecules appear to take up the energy direct, converting 

 it into their own motion, that is into heat, and where there 

 appears to be little or nothing of the nature of reflexion. I 

 am not prepared to discuss that kind at present. 



It is interesting to note that in the most resisting and 

 capacious cable that ever was made, where all the features of 

 every wave arrive as obliterated as if one were trying to 

 sigual by heat-pulses through a slab, that even there the head 

 of every wave travels imdistorted, with the velocity of light, 

 and suffers nothing but attenuation ; for the superposed re- 

 versed field is only called out by the arrival of the direct pulse, 

 and never absolutely reaches the strength of the direct field. 

 The attenuation may be excessive, but the signal is there 

 in its right time if only we have a sensitive enough instru- 

 ment to detect it ; though it would be practically useless as a 

 signal in so extreme a case, being practically all tail. 



Nothing at all reaches the distant end till the light-speed- 

 time has elapsed ; and the light-speed-time in a cable depends 

 on the /jl and K of its insulating sheath, depends, if that is 

 not simply cylindrical, on the product of its self-inductance and 

 capacity per unit length ; but at the expiration of the light- 

 speed-time the head of the signalling pulse arrives, and 

 neither wire-resistance nor insulation-leakage, no, nor mag- 

 netic-conductivity, can do anything either to retard it or to 

 injure its sharpness : they can only enfeeble its strength, but 

 they can do that very effectually. 



The transmitter of the pulse is self-induction in conjunction 

 with capacity : the chief practical enfeebler of the pulse 

 is wire-resistance in conjunction with capacity ; and before 

 Atlantic telephony is possible (unless a really distortionless 

 cable is forthcoming) the copper core of an ordinary cable 

 will have to be made much larger. Nothing more is wanted 

 in order that telephony to America may be achieved. There 

 may be practical difficulties connected with the mechanical 

 stiffness of a stout core and the worrying of its guttapercha 

 sheath, and these difficulties may have to be lessened by aiming 

 at distortionless conditions — it is well known also that for 

 high frequencies a stout core must be composed of insulated 

 strands unless it is hollow — but when such telephony is accom- 

 plished, I hope it will be recollected that the full and complete 

 principles of it and of a great deal else connected with tele- 



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