144 SIR J. F. W. HERSCHEL ON THE 



parent and colourless when held between the eye and the light, or a white object, it 

 yet exhibits in certain aspects, and under certain incidences of the light, an extremely 

 vivid and beautiful celestial blue colour, which, from the circumstances of its occur- 

 rence, would seem to originate in those strata which the light first penetrates in 

 entering the liquid, and which, if not strictly superficial, at least exert their peculiar 

 power of analysing the incident rays and dispersing those which compose the tint in 

 question, only through a very small depth within the medium. 



To see the colour in question to advantage, all that is requisite is to dissolve the 

 two ingredients above mentioned in equal proportions, in about a hundred times 

 their joint weight of water, and having filtered the solution, pour it into a tall narrow 

 cylindrical glass vessel or test tube, which is to be set upright on a dark-coloured 

 substance before an open window exposed to strong daylight or sunshine, but with 

 no cross lights, or any strong reflected light from behind. If we look down perpen- 

 dicularly into the vessel so that the visual ray shall graze the internal surface of the 

 glass through a great part of its depth, the whole of that surface of the liquid on 

 which the light first strikes will appear of a lively blue, which as the situation of the 

 eye changes is either fore-shortened into a vivid concave gleam, or opens out into a 

 paler and broader band, as the visual line is more or less oblique to the glass surface. 

 If the liquid be poured out into another vessel, the descending stream gleams in- 

 ternally from all its undulating inequalities with the same lively yet delicate blue 

 colour, thus clearly demonstrating that contact with a denser medium has no share 

 in producing this singular phenomenon. 



The thinnest film of the liquid seems quite as effective in producing this superficial 

 colour as a considerable thickness. For instance, if in pouring it from one glass into 

 another, it be made to trickle down the internal surface of the receiving glass towards 

 the light, or if instead of falling in drops from a filter, the end of the funnel be made 

 to touch the internal surface of the vessel well moistened, so as to spread the de- 

 scending stream over an extensive surface, the intensity of the colour is such that it 

 is almost impossible to avoid supposing that we have a highly coloured liquid under 

 our view. 



By candlelight the gleam is less vivid, and verges more to violet. Analysed by a 

 prism the red rays are found to be almost entirely absent. No signs of polarization 

 were perceived in it, on viewing it through a tourmaline plate turned round in its 

 own plane. 



As this phenomenon in all its circumstances is (so far as I am aware) unique in 

 physical optics, I have thought no apology necessary for simply describing, without 

 attempting to pursue it further, which present circumstances do not permit. It 

 would be interesting to know whether the property in question is characteristic of 

 quinine, or is participated in by cinchonine, salicine, or any of the other vegetable 

 alkaloids, which I have not been able to decide for want of specimens. 



J. F. W. Herschel. 

 Collwgwoody Jan, 25> 1845. 



