6/6 PHILOSOPHICAL THAN ? ACTIONS. [ANN0 1785. 



display the indigo rays. The spark however may be divided and weakened even 

 in the open air, so as to yield the most refrangible rays only. 



Exper. 11. To an insulated metallic ball, 4 inches in diameter, I fixed a wire 

 a foot and a half long. This wire terminated in 4 ramifications, each of which 

 was fixed to a metallic ball half an inch in diameter, and placed at an equal dis- 

 tance from a metallic plate, which communicated by metallic conductors with the 

 ground. A powerful spark, after falling on the large ball at one extremity of 

 the wire, was divided in its passage from the 4 small balls to the metallic plate. 

 On examining this division of the fluid in a dark room, I discovered some little 

 ramifications which yielded the indigo rays only : indeed at the edges of all weak 

 sparks the same purple appearance may be discovered. It may also be observed, 

 that the nearer we approach the centre of the spark, the greater is the brilliancy 

 of its colour. 



6. The influence of different media on electrical light is analogous to their 

 influence on solar light, and will help us to account for some very singular 

 appearances. 



Exper. 12. Let a pointed wire, having a metallic ball fixed to one of its ex- 

 tremities, be forced obliquely into a piece of wood, so as to make a small 

 angle with the surface of the wood, and to make the point lie about J- of an 

 inch below the surface. Let another pointed wire, which communicates 

 with the ground, be forced in the same manner into the same wood, so that its 

 point also may lie about ± of an inch below the surface, and about 2 inches 

 distant from the point of the first wire. Let the wood be insulated, then a 

 strong spark which strikes on the metallic ball will force its passage through the 

 interval of wood which lies between the points, and appear as red as blood. To 

 prove that this appearance depends on the wood's absorption of all the rays but 

 the red, it may be observed, that the greater the depth of the points is below 

 the surface, the less mixed are the red rays. I have been able sometimes, by 

 increasing or diminishing the depth of the points, to give the spark the follow- 

 ing succession of colours. When they were deepest below the surface, the red 

 only came to the eye through a prism. When they were raised a little nearer 

 the surface, the red and orange appeared. When nearer still, the yellow ; and 

 so on till, by making the spark pass through the wood very near its surface, all 

 the rays were at length able to reach the eye. If the points be only -±- of an inch 

 below the surface of soft deal wood, the red, the orange, and the yellow rays 

 will appear as the spark passes through it. But when the points are at an equal 

 depth in a harder piece of wood, such as box, the yellow, and perhaps the 

 orange, will disappear. As a further proof that the phenomena now describing 

 are owing to the interposition of the wood, as a medium which absorbs some of 

 the rays and suffers others to escape, it may be observed, that when the spark 



