484 EVENING DISCOURSES 



doing so let us examine the last of the three most promising vapours or 

 gases. 



This is the mercury vapour lamp with which we are now floodlighting 

 this hall. This in my opinion is the most interesting of the three, and we 

 hope to demonstrate its considerable possibilities. The first thing you 

 have noted is that it is anything but flattering to the complexion. True — 

 and that is the worst of it. 



If our complexions had no red in them and were blue or yellow it would 

 do excellently, for mercury, as we saw, has both these colours. It has also 

 violet and, still more important, ultra-violet. Red is all it really lacks. 

 We see this at once when we turn the light on to the colour chart. You 

 will note that yellows, green and blues all show well — only red is deficient. 

 Thus, light from mercury — unaided by luminescence — is better than that 

 from either sodium or from neon, on account of the greater variety of colours 

 in its light. Let us pass the colour chart across the sodium, neon and 

 mercury lights in turn. There is no doubt that mercury gives the most 

 colour. 



But one of the important advantages of mercury lies in the presence, in 

 its radiation, of plenty of ultra-violet. This you will remember was 

 radiation with a shorter wavelength than the visible blue — just too short 

 to excite the seeing functions of our eyes. This ultra-violet radiation can 

 be put to good use in exciting fluorescence. If the tube is not operated at 

 a high temperature, the fluorescent powders may be applied to its inside 

 surface, when the strong ultra-violet lines of mercury are able to exert 

 their full effect. Here is such a tube. The right half of it is coated 

 internally with a green luminescent material. The left half displays the 

 mercury discharge without the addition of luminescence. The increase in 

 light due to the fluorescence is in this case about eight times. 



Now the glass walls of the intense mercury lamps (such as all here have 

 probably seen in use for street lighting) are too hot to permit the powder 

 to be applied directly. So in this case we must apply it to an outer envelope 

 which does not become so hot. 



We now know of luminescent materials which, when stimulated by 

 ultra-violet light failing on them, will give out visible light having a red hue. 

 If we coat an outside bulb with this material the useless ultra-violet light 

 is converted by the powder into useful red light and the colour of the light 

 from the combination is thereby improved. It is not perfect, but it is 

 greatly improved. 



Here is such an outside envelope coated with powder. Underneath it 

 (and screened from you so that it does not dazzle) is an electric lamp using 

 mercury vapour. Here is a length of multi-coloured cloth placed so that 

 the light from the mercury vapour falls on it. We now lower once or twice 

 the envelope over the lamp, and you will observe how the colour renderings 

 are improved. There is no loss of efficiency when this envelope is in place 

 because the powder adds as much new light as it absorbs of the original 

 light falling on it. 



These luminescent materials bid fair to play a considerable part in 

 electric discharge lamp lighting. Whilst they have been known and used 

 for different purposes for over a hundred years we are only now discovering 

 how extremely bright they can be made, and how effectively the different 

 methods of exciting them can be employed. 



As an example of this I have an experiment here which shows how 

 powerfully these powders can be excited by the electrons present in the 

 discharge. The tube is coated internally with a powder. At present 

 there is a gas in the tube and the electrons which comprise the electricity 



