Prof. Tyndall on Luminous and Obscure Radiation. 337 



with tlie ordinary glass lenses attached to Duboscq's electric 

 lamp. They cannot, I think, fail to give pleasure to those who 

 repeat the experiments. It is extremely interesting to ob- 

 serve in the middle of the air of a perfectly dark room a piece 

 of black paper suddenly pierced by the invisible rays, and the 

 burning ring expanding on all sides from the centre of ignition. 



33. On the 15th of this month I made a few experiments on 

 solar light. The ' heavens were not free from clouds, nor the 

 London atmosphere from smoke, and at best I obtained only a 

 portion of the action which a clear day would have given me. 

 I happened to possess a hollow lens, which I filled with the con- 

 centrated solution of iodine. Placed in the path of the solar 

 rays, a faint red ring was imprinted on a sheet of white paper 

 held behind the lens, the ring contracting to a faint red spot 

 when the focus of the lens was reached. It was immediately 

 found that this ring was produced by the light which had pene- 

 trated the thin rim of the liquid lens. Pasting a zone of black 

 paper round the rim, the ring was entirely cut off and no visible 

 trace of solar light crossed the lens. At the focus, whatever light 

 passed would be intensified nine hundredfold ; still even here no 

 light was visible. 



34. Not so, however with the sun's obscure rays ; the focus 

 was burning hot. A piece of black paper placed there was in- 

 stantly pierced and set on fire ; and by shifting the paper, 

 aperture after aperture was formed in quick succession. Gun- 

 powder was also exploded. In fact we had in the focus of the 

 sun's dark rays a heat decidedly more powerful than that of the 

 electric light similarly condensed, and all the effects obtained 

 with the former could be obtained in an increased degree with 

 the latter. 



35. I introduced a plano-convex lens of glass, larger than 

 the opake lens just referred to, into the path of the sun's rays. 

 The focus on white paper was of dazzling brilliancy ; and in this 

 focus the results already described were obtained. I then 

 introduced a cell containing a solution of alum in front of the 

 focufs. The intensity of the light at the focus was not sensibly 

 changed ; still these almost intolerable visual rays, aided as they 

 were by a considerable quantity of invisible rays which had also 

 passed through the alum, were incompetent to produce effects 

 which were obtained with ease in the perfectly dark focus of the 

 opake lens. 



36. Thinking that this reduction of power might be due to 

 the withdrawal of heat by reflexion from the sides of the glass 

 cell, I put in its place a rock-salt cell filled with the opake solu- 

 tion. Behind this cell the rays manifested the power which they 

 exhibited in the focus of the opake lens. 



