14 PEOFESSOE TYNDALL ON CALOEESCENCE. 



Disks of paper reduced to carbon by different processes were raised to brilliant incan- 

 descence, both in the air and in the exhausted receiver. 



In these earlier experiments I made use of apparatus which had been constructed for 

 other purposes. The mirror, for example, was detached from a Duboscq's camera, first 

 silvered at the back, but afterwards silvered in front. The cell employed for the iodine 

 solution was also that which usually accompanies Duboscq's lamp, being intended by its 

 maker for a solution of alum. Its sides are of good white glass, the width from side 

 to side being 1'2 inch. 



§5. 



A point of considerable theoretic importance was involved in these experiments. In 

 his excellent researches on fluorescence, Professor Stokes had invariably found the 

 refrangibility of the incident light to be lowered. This rule was so constant as almost 

 to enforce the conviction that it was a law of nature. But if the rays which in the fore- 

 going experiments raised platinum and gold and silver to a red heat were wholly 

 extra-red, the rendering visible of the metallic films would be an instance of raised 

 refrangibility. 



And here I thought it desirable to make sure that no trace of the visible radiation 

 passed through the solution, and also that the invisible radiation was exclusively extra-red. 



This latter condition might seem to be unnecessary, because the calorific action of the 

 extra-violet rays is so exceedingly feeble (in fact so immeasurably small) that, even 

 supposing them to reach the platinum, their heating-power would be an utterly vanishing 

 quantity. Still mechanical considerations rendered the exclusion of all rays, of a higher 

 refrangibility than those generated at the focus, necessary to the rigid solution of the 

 problem. Hence, though the iodine employed in the foregoing experiments was sufii- 

 cient to cut ofi" the light of the sun at noon, I wished to submit its opacity to a severer 

 test. The following experiments were accordingly executed. 



A piece of thick black paper, mounted on a retort-ring, was caused gradually to 

 approach the focus of obscure rays. The position of the focus was announced by 

 the piercing of the paper ; the combustion being quenched, the retort-ring was moved 

 slightly nearer to the lamp, so that the converged beam passed through the burnt 

 aperture, the focus falling about J an inch beyond. A bit of blackened platinum held 

 immediately behind the aperture was raised to redness over a considerable space. 

 The platinum was then moved to and fro until the maximum degree of incandescence 

 was obtained, the point where this occurred being accurately marked. A cell con- 

 taining a solution of alum was then placed between the diaphragm of black paper and 

 the iodine-cell. The alum solution diminished materially the invisible radiation, but it 

 was without sensible influence on such visible rays as the concentrated beam contained. 

 All stray light issuing from the crevices in the lamp had been previously cut off", the 

 daylight also being excluded from the room. The eye was then brought on a level with 

 the aperture and slowly approximated to it, until the point which marked the focus was 



