38 LECTURES TO SCIENCE TEACH EE^. 



middle of the hole, a line of blue light for the blue light, a 

 line of red light for the red light, &c., in different positions, 

 so that the spectrum on the screen would be made up of a 

 series of lines, red, yellow, and blue, &c., if there were so 

 many different kinds of light. Every pure spectrum from a 

 source of light allowed to fall through a slit is to be thought 

 of as made up of a number, generally an infinite number, of 

 images of the . slit, corresponding each to the light of one 

 definite refrangibility. Instead of having distinct images of 

 the slit, as you would if there were only a definite number 

 of degrees of refrangibility, if you have all shades you must 

 regard the spectrum as made up of an infinite number of 

 images of the slit placed side by side, and running one into 

 the other with no line of demarcation between them, except 

 in certain conditions in which there is a failure of certain 

 kinds of light. Now the simplest way of seeing this is to 

 take a slit, which may be of the roughest description, and a 

 prism which is just large enough to cover comfortably the 

 pupil of the eye, and to look at the slit through the prism as 

 I am doing now. I now see a coloured image or spectrum, and 

 in it the fixed lines of Fraunhofer. T presume you have heard 

 of these already, and I shall not describe them, as it would 

 take me too far from my subject. There is one point which I 

 must notice in the use of the prism held in this manner before 

 the naked eye. If you turn it round its axis, you will find 

 that for a certain azimuth of the prism the fixed lines of the 

 spectrum, or at least of a particular part that you are looking 

 at, will be seen distinctly ; but if you turn the prism a little this 

 way or that round a line parallel to its edge, they will become 

 indistinct. The particular direction or azimuth in which the 

 prism must be held is found by trial. You can focus the 

 spectrum by turning the prism one way or the other, until 

 the image you are looking at is sharp and clear; just as 

 in ordinary cases of focusing. The reason of that is easily 

 explained by geometrical optics, or the science which treats 

 of the mathematical consequences of the laws of reflection and 

 refraction of rays of light. It depends on the alteration of 

 the distance of the virtual focus from which the rays after 

 refraction through the prism come according to the azimuth 

 of the prism. I say virtual focus, although in point of fact, 

 after passing in that manner through a prism, unless it be in 

 the position of minimum deviation, the light diverges, not from 



