40 LECTURES TO SCIENCE TEACHERS. 



prismatic image of a candle, and then, keeping that in the 

 field, move off the prism to arm's length, so as to get distinct 

 vision of the prism itself, by moving it about a little you 

 will be able to see whether it is free from veins or not. It 

 should be free from veins, although a prism for merely eye- 

 work need not be of the same excellence as if it were to be 

 used with a telescope. 



. The more immediate object of my lecture is the coloration 

 of natural objects, and that is best studied in the first instance 

 in the case of clear coloured bodies such as solutions, or 

 coloured glasses. Here is a coloured solution, and if I reflect 

 the skylight through it you will see the colour is blue ; but 

 if I add a little more colouring matter to it, it is no longer 

 blue but red. The same effect exactly would be produced if, 

 instead of increasing the quantity of coloured fluid which was 

 mixed with the water, I had increased the thickness of the 

 stratum through which you looked in fact one is found by 

 experiment to have exactly the same effect as the other. What 

 is the colour of this fluid 1 If you saw it only in one stage 

 you would say it was blue, and if only in another you 

 would say it was red. It passes in fact from blue to red. 

 "What is the cause of that 1 You must remember that this fluid 

 is illuminated by white light, and white light is not all of the 

 same kind, but is a mixture of portions of light, differing from 

 one another by their refrangibility, and at the same time 

 differing from one another in the coloured impression which 

 they produce upon the eye. In glisses the colouring effect 

 upon the eye simply results from the super-position of various 

 kinds of light which are present. In order to study this 

 phenomenon and the cause of it, we must in the first instance 

 consider what would take place as regards one kind of .light 

 alone. If I had one kind of light (and approximately I 

 should get that by a Bunsen flame with a bead of common 

 salt introduced into it), supposing I viewed this through a 

 wedge-shaped vessel which I can slide in front of my 

 eye, if I begin where the thickness is nothing, no effect is 

 produced. In this particular fluid, if I had such a flame before 

 me, I should see at first no effect, and then, as I slid the vessel 

 so as to increase the thickness of fluid looked through, the 

 flame would become weaker and weaker, until I should not 

 be able to see it. The effect is one of a progressive weaken- 

 ing. The longer the path, of the light within this coloured 



