322 Notes and Memoranda. 



modifications of the same thing, it could scarcely be considered as demonstrable 

 by experiment. Professor Tyndall has, however, now succeeded in doing this. 

 He was led to the important results he has attained by the discovery, a few years 

 since, of a medium perfectly pervious to radiant heat, but at the same time per- 

 fectly opaque to light. To effect the change of heat into light he obtains a cone 

 of convergent rays by concentrating the rays of the electric light with a small 

 mirror, and then intercepts all those "which are luminous by an opaque solution of 

 iodine in bisulphide of carbon, which allows all the calorific rays to pass on to a 

 focus. The latter, even in a darkened room, is completely invisible ; but any 

 combustible substance placed within it is immediately inflamed, as described in 

 our number for March, p. 161. If this combustible is zinc, or magnesium, for 

 example, the light produced is intensely brilliant. To render it impossible that 

 its luminous character should be due to the process of combustion, platinum was 

 thus heated in vacuo, and the light emitted produced a perfect spectrum. The 

 invisible picture of .the charcoal points, so to speak, was changed into a visible by 

 decreasing the refrangibility of the rays — that is, by transformation of the heat 

 into light ; and every change made in the position of these points was distinctly 

 perceptible on the platinum. This may be looked upon as the completion of 

 what was begun thirteen years ago by Professor Stokes. The latter rendered the 

 ultra-violet rays visible by diminishing their refrangibility ; Professor Tyndall 

 has rendered the invisible rays belonging to the opposite extremity of the 

 spectrum visible by increasing theirs. A similar proof of the identity of light 

 and heat was, indeed, suggested by Aikin at the meeting of the British Associa- 

 tion in 1863 ; but he proposed to use sun light, and has not as yet been suc- 

 cessful, though he employs a mirror three feet in diameter. Professor Tyndall, 

 by using the electric light, attained his object, and with the use of a mirror only 

 four inches in diameter. 



The Microscope and Spectroscope Combined. — Mr. Sorby places a small 

 prism beneath an achromatic condenser, and allows light to fall upon it through a 

 slit placed at a certain distance in front of a lamp. Exceedingly dilute solutions 

 of blood and certain other substances being interposed, produce the dark 

 bands discovered by Professor Stokes, and when the spectrum is viewed through 

 the microscope, with a low power, they are readily seen. Mr. Browning has 

 carried out Mr. Sorby's idea in a very neat and convenient way ; but we doubt 

 whether the intervention of a microscope will prove to be the most advantageous 

 mode of viewing very delicate absorption bands. We made some experiments at 

 Mr. Browning's, which suggest the correctness of this view. First we reduced 

 by further dilution a very weak solution of blood in water, until the absorptive 

 action on the prismatic spectrum was very faintly seen under the microscope. The 

 solution was then examined by a large and fine single prism spectroscope, having 

 considerable dispersion, and no absorption band could be seen. Mr. Browning at 

 once suggested the true cause, viz., too much dispersion. A smaller spectroscope, 

 with less dispersive power, was then tried, and the absorptive action at once became 

 visible. It is evident that while the separation of closely adjacent lines in spectra 

 can only be affected by considerable dispersion, a large amount of that kind of 

 action thins out, and renders invisible, very delicate absorption bands. A spec- 

 troscope to show such bands should have such an angle as to produce little dis- 

 persion, and the spectrum should be viewed with slight magnification, or with 

 none at all. 



Light Streaks in the Ceisian Sea. — Mr. Slack states that on the 2nd of 

 April, in the evening, when the moon was about 6 days 14 hours old, the 

 Crisian Sea presented an unusual appearance, being striped with delicate bands of 

 light radiating from the vicinity of a small mountain near Picard. Several light 

 spots were also noticed in parts of the same sea, which are usually dark. Mr. 

 Slack inquires whether other observers noticed these appearances. 





