Dr. C. K. Akin on Calcescence. 41 



ocular diaphragm when the red glass was used as an objective 

 diaphragm, was deep-green glass. Pieces of such glass were used 

 for insertion within the angular tube near the end where the eye 

 was placed, and were made moveable from without ; so that the 

 incandescence of the platinum, when produced, could be viewed 

 either freely or through the ocular diaphragm. 



It will be seen that our arrangements were pretty well consi- 

 dered. Unfortunately, when our preparations were nearly ma- 

 tured, my collaborator, Prof. Griffith, as it appeared, was not 

 able to give to the subject so much of his time and attention as 

 it required, and towards August this year the experiments were 

 consequently allowed to drop unfinished. 



(ii) I have stated in a former Number of the Philosophical Maga- 

 zine my probable inability to pursue the subject-matter of this 

 paper any further, at least experimentally*. Meanwhile I will 

 take this occasion to make some remarks concerning experiments 

 on artificial sources of rays. 



Our first trials at Oxford were made with a couple of brass 

 reflectors and the flame of the oxyhydrogen jet, in the manner 

 before described; but, no doubt in a great measure from the 

 imperfect condition of the reflectors, the trials were not successful. 

 It is evident that, instead of a couple of reflectors, a single re- 

 flector might be made to render practically the same service, the 

 flame and the object to be rendered incandescent being respect- 

 ively placed at any two conjugate foci of the reflector. The oxyhy- 

 drogen jet has the advantage in these experiments of being 

 comparatively deficient in Newtonic and Ritteric rays, which 

 might hence be easily eliminated altogether without much detri- 

 ment to the Herschellic rays. On the other hand, gases are 

 known to be but indifferent radiators as compared with solid 

 bodies, as was shown long ago by Mellonif. Hence it might 

 be preferable perhaps to use lime-light for the purposes here in 

 question, instead of the pure oxyhydrogen-flame, as possibly the 

 Herschellic radiation of lime-light remaining after the Newtonic 

 and Ritteric rays have been eliminated by proper absorbents might 

 surpass in intensity the similar radiation emanating from the 

 pure oxyhydrogen-flame. 



Lampblack having been shown by Melloni to be a better absorb- 

 ent of nearly all kinds of radiations than chalk or lime J, incandes- 

 cent coal is presumably a more prolific source of Herschellic rays, 

 the temperature being the same, than incandescent lime. For 

 this reason, coal or coke, rendered incandescent either in the 



* Since this paper was put in type, I have been led to hope, by author- 

 ized members connected with a great scientific society, that means will be 

 found to set my experiments on foot again. 



t La Thermochrose, p. 94. % Ibid. p. 96. 



