loith the Brush Discharge. 55 



comfort, however, to know that there is considerable difficulty 

 in recognizing the words owing to the folding of the paper, 

 and thus one word coming immediately on the top of another. 

 Fig. 5 shows the result obtained when a printed invitation- 

 card enclosed in an envelope was exposed to the brush- 

 discharge, the envelope being placed on the sensitive film. 

 Notice the texture of the paper, and the opacity caused by 

 the gum and double thickness of paper. 



Light from the Discharge. 



The great similarity between the effects recorded in the 

 foregoing pages and those produced by ordinary light led to 

 experiments being made to compare them, 



The first point which suggested itself was to see whether the 

 interposition of a body which was transparent to ordinary 

 light, between the point and plate, made any difference in the 

 result. A plate of clear glass was used, and the shadow of 

 an object obtained. No difference was observed in the sharp- 

 ness of the image. The glass was now blackened with lamp- 

 black until it was so opaque to light that the flame of an 

 ordinary candle could not be seen through it when it was 

 held at a distance of 5 centims. from the flame. When this 

 was held between the point and the photographic plate no 

 effect whatever could be obtained. 



Again, the law of inverse squares was proved in the follow- 

 ing manner : — A small cross cut out of thin sheet-zinc was 

 placed at a certain distance from the sensitive plate and its 

 shadow obtained. The plate was now moved so that the 

 shadow should fall upon a different part of the film, and the 

 object was placed at a different distance away. Another 

 shadow was now obtained. This was of a different size to 

 the former. The two figures were then measured, and the 

 sizes compared with the object and the distance of the brush. 



These results therefore indicate that the effects are produced 

 by the light which the brush emits. Moreover, after many 

 trials, I have been enabled to reproduce all the effects with 

 artificial sources of light. Many of them can be produced by 

 employing daylight, and probably all could thus be formed, 

 but the length of exposure required has hitherto prevented 

 this from being done. 



But, notwithstanding this apparently simple explanation, it 

 is quite possible that we have something else taking place at 

 the same time. 



Suppose that the point is the centre of a disturbance from 

 which waves are emitted. These waves will be of various 

 lengths, some capable of affecting the optic nerve, and some 



