56 LECTUEES TO SCIENCE TE AGREES. 



with sulphuric acid, by transmitted light the fluid looks very 

 much like water, but it exhibits in certain aspects a blue 

 colour. You will not perhaps very well see it here, but this 

 is such a common fluid, easily obtained by anyone, that it is 

 almost sufficient to mention the appearance. What is 

 remarkable about this blue colour is that (unless the solu- 

 tion be excessively dilute) it is mainly concentrated, and 

 occurs in an exceedingly narrow stratum adjacent to the 

 surface by which the light enters the fluid. This diagram 

 [referred to] represents the appearance. 



This [referring to figure] is supposed to be a section of a 

 tumbler containing the solution, placed on a black ground, 

 and tolerably near a window from which light is coming in 

 approximately horizontally. "When you look down from 

 above you see that the side of the fluid next the window 

 is marked by this bluish colour, and when you hold the eye 

 almost in a prolongation of the anterior surface of the fluid 

 you see this blue stratum very much foreshortened and 

 thereby increased in intensity, because the fluid itself is 

 transparent like water, and the blue light which appears, 

 whatever may be its cause, is seen perfectly well through 

 it. When you look down in an oblique direction you see it 

 much less intense. It is seen in perfection on allowing the 

 light to shine from above, holding the eye a shade below the 

 level of the upper surface, and putting a black object to 

 make a dark background. 



Now what is the nature of this blue light] Sir John 

 Herschel tried various experiments on it. He analysed it 

 by a prism, and obtained a continuous spectrum. He 

 noticed, however, that he did not see the Fraunhofer lines 

 in the spectrum ; but whether they were really absent, or 

 that he did not see them because the light was not strong 

 enough, he does not seem quite decided. He noticed also 

 that the blue light exhibited no trace of polarisation. He 

 examined further the light transmitted through the solution 

 to see what blue rays were taken out of it. Naturally he 

 was led to scrutinise more particularly the blue part of 

 the spectrum, but apparently the blue part of the spectrum 

 was like the blue part of the spectrum of light which had 

 come through simple water ; there was nothing particular 

 to be seen in it to account for the phenomenon. Possibly 

 ovever, if this superficial colour is produced once only, 



