34 Dr. C. K. Akin on Calcescence. 



necessary that the platinum should become incandescent as here- 

 tofore, when irradiated by the whole cone of rays which is found 

 concentrated at the focus of the mirror. An experiment of this 

 nature (which supposes besides that /3 by itself is not competent 

 to produce incandescence) would be open, however, to exception, 

 in consequence of the dependence of electrical resistance on tem- 

 perature ; but it might be varied in the following manner, in 

 which it would become entirely unexceptionable. The apparatus 

 being all arranged as above, let an opake screen be at first inter- 

 posed between the mirror and the platinum in its focus ; but, on 

 the other hand, let the platinum be rendered incandescent by 

 the agency of the galvanic battery alone with which it is con- 

 nected. Then, breaking this connexion, let the moment be 

 seized at which the platinum, though still hot, ceases to shine, 

 and let at the same instant the screen mentioned above be removed, 

 so as to allow the rays reflected from the mirror to impinge upon 

 the platinum, after having traversed the red diaphragm. These 

 rays being of sufficient calefactory power (as found upon trial) 

 to burn dry paper and pieces of wood, will presumably be com- 

 petent to revive the incandescence of the platinum, if dexterously 

 applied at the very moment of its extinction. The experiment 

 being made either in the first way or in that just described, it 

 would be necessary to observe whether the incandescence pro- 

 duced is visible if looked at through an eye-glass of such a nature 

 as to be certain that all rays of equal or less refrangibility than 

 those transmitted by the red glass are absorbed, whilst only such 

 as are of higher refrangibility are permitted to reach the observer. 

 Such a medium might be found in certain varieties of green glass, 

 which absorb all the rays of the spectrum of lower refrangibility 

 than the lineD — supposing the so-called monochromatic red glass 

 transmitted only these very rays, and none of greater refrangi- 

 bility. This is generally supposed. On looking, however, at 

 the sun with a pocket spectroscope through the very deepest red 

 glass, it was easy to notice that, besides the rays mentioned, it 

 transmitted, under the circumstances, rays of greater refrangi- 

 bility, extending towards the blue region of the spectrum, but 

 separated from the red and orange by a perfectly dark band; so 

 that it became at least doubtful whether this and other red glass, 

 on being employed for the above experiment in the way described, 

 might not allow rays of higher refrangibility to impinge on the 

 platinum than the green glass is capable of absorbing. The in- 

 vestigation of this point and other untoward circumstances did 

 not allow of the experiments suggested to be performed in time 

 to be communicated, except as a project, to the Association*. 



* The Association have since placed at the disposal of the author and Mr. 

 G. Griffith a grant of money to carry out the experiments suggested by him 

 with a more powerful mirror than was employed in the above-described trials. 



