PEOFESSOE TYNDALL ON CALOEESCENCE. 15 



reached. A singular appearance presented itself. The incandescent coke points of the 

 lamp were seen perfectly black, projected 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 incan- 

 descent 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 constructed — the one with transparent rock-salt sides, 

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

 nearly 2| inches. Filled with the solution of iodine, these 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 aperture 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 The eye was then withdrawn, and a plate of platinized platinum was placed in the posi- 

 tion occupied by the retina a moment before. It instantly 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 collect as many of the invi- 

 sible 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 mirror, 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 con- 

 siderable 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 annealing 

 repeatedly occurred ; but at length I was so fortunate as to obtain the three mirrors, each 



* I do not recommend the repetition of these experimenta. 



