G8 LECTURES TO SCIENCE TEACHERS. 



examine it by a prism applied to the eye, you will, however, 

 see that the violet, or more or less of the violet, is gone ; 

 instead of seeing as is usual the double line H, you will see 

 the fluid terminate, according to the strength of the solution 

 and the thickness looked through, more or less towards the 

 violet, say on an average about half way between the fixed 

 Knes G and H. 



IsTow the incident rays work on the fluid in such a 

 manner as to cause it to give out light of a different kind 

 altogether ; a light which is found, to be heterogeneous, or 

 to consist of rays of various degrees of refrangibility. This 

 rule I find to be universal, namely, that the refrangibility 

 of the light in this process is always lowered. I have 

 never found any exception to that, nor I believe has anyone 

 since. 1 The rays which any one of these fluids is capable 

 of giving out under the influence of the^e other rays aie 

 always of lower refrangibility, and you never have the 

 refrangibility raised. 



I will endeavour presently to show a test-tube with one 

 of these solutions in part of the spectrum, though I cannot 

 promise that it will be seen at a distance. The fact is I 

 am accustomed to work with sunlight rather than with the 

 electric light and I require more preliminary trials than I 

 allowed myself for making the thing succeed. Still I think 

 you see that on interposing a test-tube with the solution of 

 quinine in the beam from the electric lamp, after it has 

 passed through the prism, it cuts off certain portions of the 

 spectrum thrown on the wall beyond, forming a shadow 

 which shows in what part of the rays proceeding to form 

 the spectrum the tube is for the moment placed ; the blue 

 light with which the solution glows, commencing about the 

 violet, is seen altogether beyond the region of the visible 

 rays. Here is a solution of a substance obtained from the 

 bark of the horse-chestnut which shows it still better. You 

 observe the blue band beyond the visible spectrum altogether. 

 Another instance is when we allow the beam of light to fall 

 on a piece of red cloth, it shows an orange band beyond the 

 visible rays. 



1 Calorescence, or tho exhibition of light by a body intensely heated 

 by .the concentration upon it of invisible heat-rays, is in some respects 

 so different from the phenomena of fluorescence or phosphorescence that 

 I do not regard it as forming any exception to the rule. 



