438 Prof. Tyndall on Calorescence. 



reflected rays issued, forming a focus outside the camera. 

 Underneath this aperture was riveted a stage, on which the solu- 

 tion of iodine rested, closing the aperture and cutting off all the 

 light. In the first experiments nothing intervened between the 

 cell and the carbon-points; but the peril of thus exposing the 

 bisulphide caused me to make the following improvements. 

 First, a perfectly transparent plate of rock-salt, secured in a 

 proper cap, was employed to close the aperture ; and by it all 

 direct communication between the solution and the incandescent 

 carbons was cut off. The camera itself, however, became 

 quickly heated by the intense radiation falling upon it, and the 

 cell containing the solution was liable to be warmed both by the 

 camera and by the luminous rays which it absorbed. The aper- 

 ture above referred to was therefore surrounded by an annular 

 space, about 2\ inches wide and a quarter of an inch deep, 

 through which cold water was caused to circulate. The cell 

 containing the solution was moreover surrounded by a jacket, 

 and the current, having completed its course round the aperture, 

 passed round the solution. Thus the apparatus was kept cold. 

 The neck of the cell was stopped by a closely-fitting cork ; through 

 this passed a piece of glass tubing, which, when the cell was 

 placed upon its stage, ended at a considerable distance from the 

 focus of the mirror. Experiments on combustion might there- 

 fore be carried on at the focus without fear of igniting the small 

 amount of vapour which even under the improved conditions 

 might escape from the bisulphide of carbon. The arrangement 

 will be at once understood by reference to Plate IV. figs. 4 a & £, 

 which show the camera, lamp, and filter both from the side and 

 from the front, oc y is the mirror, from which the reflected cone 

 of rays passes, first, through the rock-salt window (unshaded), 

 and afterwards through the iodine filter mn. The rays converge 

 to the focus k, where they would form an invisible image of the 

 lower carbon-point ; the image of the upper would be thrown 

 below k; and both images spring vividly forth when a leaf of 

 platinized platinum is exposed at the focus. At ss (Plate IY. 

 fig. 4 a) is shown, in section, the annular space in which the 

 cold water circulates. Fig. 4 b (Plate IV.) shows the manner in 

 which the water enters this space and passes from it to the 

 jacket surrounding the iodine-cell m. 



With this arrangement, and a battery of fifty cells, the fol- 

 lowing results were obtained : — 



A piece of silver-leaf, fastened to a wire ring and tarnished by 

 exposure to the fumes of sulphide of ammonium, being held in 

 the dark focus, the film flashed out occasionally into vivid redness. 



Copper-leaf tarnished in a similar manner, when placed at 

 the focus, was raised to redness. 



