274 KELVIN 



nary chemical or photographic effect. Photographers can 

 leave their usual sensitive chemically prepared plates ex- 

 posed to yellow light and red light without experiencing 

 any sensible effect; but when you get toward the blue end 



FIG. 120 The Solar Spectrum 



of the spectrum the photographic effect begins to tell, and 

 more and more strongly as you get towards the violet end. 

 When you get beyond the violet there is the invisible light 

 known chiefly by its chemical action. From yellow to violet 

 we have visual effect, heating effect, and chemical effect, all 

 three ; above the violet only chemical and heating effects, and 

 so little of the heating effect that it is scarcely perceptible. 



The prismatic spectrum is Newton's discovery of the 

 composition of white light. White light consists of every 

 variety of colour from red to violet. Here, now, we have 

 Newton's prismatic spectrum, produced by a prism. I will 

 illustrate a little in regard to the nature of colour by putting 

 something before the light which is like coloured glass ; it is 

 coloured gelatin. I will put in a plate of red gelatin which 

 is carefully prepared of chemical materials and see what that 

 will do. Of all the light passing to it from violet to red it 

 only lets through the red and orange, giving a mixed reddish 

 colour. Here is a plate of green gelatin: the green absorbs 

 all the red, giving only green. Here is a plate absorbing 

 something from each portion of the spectrum, taking away 

 a great deal of the violet and giving a yellow or orange ap- 

 pearance to the light. Here is another absorbing the green 

 and all the violet, leaving red, orange, and a very little faint 

 green. 



When the spectrum is very carefully produced, far more 

 carefully than Newton knew how to show it, we have a 

 homogeneous spectrum. It must be noticed that Newton did 

 not understand what we call a homogeneous spectrum ; he did 

 not produce it, and does not point out in his writings the 

 conditions for producing it. With an exceedingly fine line of 

 light we can bring it out as in sunlight, like this upper picture 



