111.1 



DIRECT-VISION PRISMS. 



27 



arrange them with their bases the opposite way, the deviation 

 caused by the one prism in the one direction, will be neutralised 

 by the deviation of the two prisms in the opposite direction ; 

 whilst the dispersion by the two prisms exceeds that which is 

 caused by the one prism in the opposite direction ; the latter 

 dispersion, therefore, will neutralise a portion only of the dis- 

 persion due to the two prisms. The final result is that there is 



FIG. 10. Direct-vision prism with three prisms, showing path of ray. 



an outstanding dispersion after the deviation has been neutralised ; 

 so that when we want to examine the spectrum of an object we 

 no longer have to look at it at an angle, but by this arrangement 

 we have an opportunity of seeing the spectrum of an object by 

 looking straight at the source of light. 



FIG. 11. Direct-vision prism with five prisms. 



If we now pass from the improved methods already intro- 

 duced by Wollaston and Fraunhofer to the phenomena they 

 observed whether in solar light, or the terrestrial light sources 

 which they examined, we find revelations of the first order of 

 importance. 



The candle which was used in our first experiment (page 9) 



