Prof. Tyndall on Calorescence. 441 



cells were placed in succession in front of the camera, and the 

 concentrated beam was sent through them. Determining the 

 focus as before, and afterwards introducing the alum-cell, the 

 eye on being brought up to the focus received no impression of 

 light. The alum-cell was then abandoned, and the undefended 

 eye was caused to approach the focus : the heat was intolerable, 

 but it seemed to affect the eyelids and not the retina. An aper- 

 ture somewhat larger than the pupil being made in a metal 

 screen, the eye was placed behind it, and brought slowly and 

 cautiously up to the focus. The whole concentrated beam here 

 entered the pupil ; but no impression of light was produced, 

 nor was the retina sensibly affected by the heat. The eye was 

 then withdrawn, and a plate of platinized platinum was placed 

 in the position occupied by the retina a moment before. It in- 

 stantly rose to vivid redness*. The rays which produced this 

 incandescence were certainly invisible ones, and the failure to 

 obtain, with the most sensitive media and in the darkest room, 

 the slightest evidence of fluorescence at the obscure focus, proved 

 the invisible rays to be exclusively extra-red. 



When intense effects are sought after, the problem is to col- 

 lect as many of the invisible rays as possible, and to concentrate 

 them on the smallest possible space. The nearer the mirror is 

 to the source of rays, the more of these rays will it intercept and 

 reflect, and the nearer the focus is to the same source, the smaller 

 will the image be. To secure proximity both of focus and mir- 

 ror, the latter must be of short focal length. If a mirror of 

 long focal length be employed, its distance from the source of 

 rays must be considerable to bring the focus near the source, 

 but when placed at a distance, a great number of rays escape 

 the mirror altogether. If, on the other hand, the mirror be 

 too deep, spherical aberration comes into play ; and though a 

 vast quantity of rays may be collected, their convergence at the 

 focus is imperfect. To determine the best form of mirror, I 

 had three constructed: the first is 4*1 inches in diameter, and 

 of 1*4 inch focal length ; the second 7*9 inches in diameter, and 

 of 3 inches focal length ; the third 9 inches in diameter, with a 

 focal length of 6 inches. Fractures caused by imperfect anneal- 

 ing repeatedly occurred ; but at length I was so fortunate as to 

 obtain the three mirrors, each without a flaw. The most con- 

 venient distance of the focus from the source, I find to be about 

 5 inches ; and the position of the mirror ought to be arranged 

 accordingly. This distance permits of the introduction of an 

 iodine-cell of sufficient depth, while the heat at the focus is 

 exceedingly powerful. 



The isolation of the luminiferous aether from the air is stri- 

 * I do not recommend the repetition of these experiments. 



