440 Prof. Tyudall on Calorescence. 



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 sufficient to cut off 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 half 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 oc- 

 curred being accurately marked. A cell containing 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 

 reached. A singular appearance presented itself. The incan- 

 descent coke-points of the lamp were seen perfectly black, pro- 

 jected on a deep-red ground. When the points were moved up 

 and down, their black images moved also. When brought into 

 contact, a white space was seen at the extremities of the points, 

 appearing to separate them. The points were seen erect. By 

 careful observation the whole of the points could be seen, and 

 even the holders which supported them. The black appearance 

 of the incandescent portion of the points could of course only 

 be relative; they intercepted more of the light reflected from 

 the mirror behind than they could make good by their direct 

 emission. 



The solution of iodine, 1*2 inch in thickness, proving unequal 

 to the severe test here applied to it, I had two other cells con- 

 structed — the one with transparent rock-salt sides, the other with 

 glass ones. The width of the former was 2 inches, that of the 

 latter nearly 2^ inches. Filled with the solution of iodine, these 



