60 LECTURES TO SCIENCE TEACHEK& 



endeavour to show you the fundamental experiment of Sir 

 John Herschel, that this epipolic dispersion is a thing which 

 cannot be repeated ; once done the light is shorn of the 

 power of producing it. I send a beam emanating from the 

 electric lamp horizontally, and reflect it downwards on the 

 surface by a mirror. There is a second vessel of the solu- 

 tion which I place floating in the other, and the blue 

 stratum is seen at the upper surface of the fluid in the 

 upper vessel, while the quinine in the lower vessel shows 

 nothing of it. There is still the general diffused light, but 

 this intense blue stratum is wanting altogether beneath the 

 bottom of the upper vessel. Now I will replace the upper 

 beaker by one of water, when you will see the difference. 

 Just below the bottom of the upper vessel you have this 

 intense blue stratum, whereas the light passing through the 

 water in the upper beaker shows nothing of the kind. That 

 is evidence that when the blue stratum is formed once, the 

 light is shorn of the power of forming it again. 



As the room is darkened I will show one or two more 

 examples of highly fluorescent solutious before I pro- 

 ceed. Here is a solution obtained in a particular manner, 

 which I will mention by and by, from the bark of the horse- 

 chestnut. 



I told you what occurred to me as to the cause of the 

 phenomenon, and it was easy to test it I will not go 

 through all the experiments I tried, but pass at once to 

 what you may regard as the fundamental experiment. 

 Suppose we form a pure spectrum in the ordinary way; that 

 we reflect the sunlight horizontally into a darkened room, 

 passing it through a slit, and at a distance from the slit 

 place one or more prisms combined with a lens, so as to 

 form at the focus of the lens conjugate to the slit a pure 

 spectrum. If you were to receive the pure spectrum on a 

 screen, you would see the various colours, and if it were 

 pure enough you would see the principal Fraunhofer lines. 

 Now suppose instead of a screen you receive it on this 

 colourless fluid. The appearance is rudely represented on 

 this diagram. 



I must mention that portions of the diagram are merely 

 diagrammatic, namely, the top and bottom. The middle 

 part represents what you actually see when you look down 

 on the solution, but the top represents what you would see 



