350 • Me Stevenson on a new Signal- Light for Railways. 



of range would be produced without the necessity of inclining , 

 the lens ; but the principle of rendering the signal at all times 

 effective, by combining a short range and a powerful light, is 

 the same in both arrangements. 



The advantage of this arrangement I conceive to be great, 

 for not only would the light be at all times visible to the en- 

 gineman on his arrival at the same point, which, as already 

 mentioned, is really the great object of signal-lights ; but it 

 is obvious that his attention would be most effectually awak- 

 ened by the contrast of suddenly passing from darkness to 

 receive the full effect of a powerful light viewed from a short 

 distance. One other advantage of the proposed signal-fight, 

 I must observe, lies in its being peculiarly susceptible of any 

 modification of colour, whether of a temporary or permanent 

 kind, which the numerous and growing wants of an extended 

 railway-system may require. The alphabet of nocturnal tele- 

 graphy, wherever a distant range is required, is unhappily 

 extremely scanty ; for the practice of all Europe seems to have 

 shewn that, ro far as colour is concerned, red and white are its. 

 alpha and omega : green and blue have been frequently tried ; 

 but cautious inquirers have all agreed in pronouncing them so 

 equivocal when viewed from a distance, that they have been 

 abnost universally abandoned. These colours, however, and 

 even much less marked varieties, although useless as distinc- 

 tions for lights of distant range, are perfectly effective when 

 viewed from short distances, as the brilliant display of an 

 apothecary's window sufficiently proves. 



I shall now add a very few words regarding what appears 

 to me to be the chief arrangements which may in practice be 

 found necessary for signal-lights on these principles ; but I 

 would not be understood as attempting to fix any thing per- 

 manently, for I am well aware that various modifications may 

 be suggested by experiment, which I do not at present foresee 

 in their full extent ; in particular, it seems probable that the 

 range of visibility which I have adopted in the following view 

 of the details falls short of what will be found quite sufficient 

 in practice even during the thickest fogs, when a light so 

 powerful as that which may be derived from FresneFs lens 



