200 Prof. Tyndall on Radiation 



j=E top of the spectrum-plate in Fresenius's Zeitschrift, 

 ■^E* is approximative^ given in the accompanying diagram, 

 by help of which our results may be directly compared 

 with those of KirchhofF and Bunsen. 



The order of brilliancy in the lines of what we sup- 

 pose to be the spectrum of pure caesium, with but the 

 minutest trace of sodium, is for the red lines as follows : 

 VI. II. VII. I. V. III. IV. The line IV. is only 

 made out under the most favourable conditions. II., 

 nearly coincident with a lithium of KirchhofF and 

 Bunsen, and not figured by them, is as bright as their 

 y caesium, our VI. (?) Among the yellow and green 

 lines to the right of the sodium line, the order of 

 brilliancy is the following : VIII. IX. XI. XII. XIV. 

 XIII. XV. X. The yellow line VIII. is hardly less 

 characteristic of the spectrum of pure caesium than the 

 two blue lines. It also is nearly as distinct as any of 

 the green lines when sodium is not present in too large 

 quantity, and is much more readily made out than 

 the extreme red line 8 of rubidium. 



To sum up, we find four red lines to the left of those 

 given by KirchhofF and Bunsen, one of which is as 

 bright as any of the red lines in the caesium spectrum. 

 Further, the red lines of KirchhofF and Bunsen are not 

 figured in their true positions, being too near each other 

 and too far to the right. Finally, we observe a fine yel- 

 low line and two unimportant green lines not mapped 

 by them. The lines which we have supplemented to 

 those of KirchhofF and Bunsen are not characteristic 

 except in the absence of foreign matters. For this 

 very reason, however, they become important to those 

 who are engaged in the study of the new elements. 

 New Haven, Conn., December 24, 1862. 



XXVII. On Radiation through the Earth's Atmosphere, By 

 John Tyndall, F.R.S., Professor of Natural Philosophy, 

 Royal Institution*. 



NOBODY ever obtained the idea of a line from Euclid's 

 definition that it is length without breadth. The idea is 

 obtained from a real physical line drawn by a pen or pencil, and 

 therefore possessing width, — the idea being afterwards brought, 

 by a process of abstraction, more nearly into accordance with the 

 conditions of the definition. So also with regard to physical 



* From the Proceedings of the Royal Institution for Jan. 23, 1863. 



























































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