.*U8 Intelligence and Miscellaneous Articles. 



By pulling a lamp in the place of the slit of the spectroscope 

 and following the rays which have passed through the photometer, 

 one can see what points of the space can send light into the spec- 

 troscope, and consequently regulate the apparatus. These rays 

 form a condensed pencil with a diameter of less than 5 millims. 

 and length of 20 centims. 



The flames which I employed were produced in the manner 

 following. Air compressed to 1*5 atmosphere issues through an 

 adjutage, above which is fixed the tapering end of a glass tube, 

 the other extremity of which dips into a saline solution ; the solu- 

 tion is aspirated and reduced to a very fine powder. The illumi- 

 nating gas is aspirated at the same time, mixes with the air and 

 the saline powder, and burns above a metallic gauze. The blue 

 part of the flame (which gives the lines of carbon) rises 2 or 

 3 millims. above the gauze ; the flame is afterwards homogeneous 

 and but little reducing ; it forms a cone 8 centims. in height by 

 2 in diameter. It was towards the base that my observations were 

 made in the experiments, of which I have now to speak. 



The object of these was to seek the mean intensity of the rays 

 which constitute the lines of the spectrum of coloured flames. Let 

 7c be a constant, i the intensity of the ray of wave-length X, X x and 

 A 2 two wave-lengths selected on one side and on the other of the 

 line considered, the brightness of this line will be equal to 



I=Jc\ id\. 

 Jax 



If I place near the flame a mirror of reflecting-power E, arranged 

 so that the same part of the flame sends its light into the photo- 

 meter direct and after reflection, the reflected rays will meet with 

 the flame at the place to which they have been emitted ; the bright- 

 ness of the line becomes 



r\ 2 r\ 2 



r=(l + K)fcl id\-Wc) i 



c/'Aj c^Ai 



supposing that the unit chosen was the intensity of the ray of the 

 same wave-length, emitted by a substance with absolute absorptive 

 power and raised to the temperature of the flame, and applying the 

 principle of the equality of the powers of absorption and emission, 

 whence 



^a 2 



i(i+E)-r 



IJa 



2EI 2 f *? A 



1 



this is the ordinate of the centre of gravity of the area included 

 between the axis of the A's and the curve formed by erecting at each 

 point of this axis an ordinate equal to i. This height of the centre 

 of gravity may vary from to \ ; and its value indicates which are 

 the dominant radiations. 



The experiments have shown that the narrow lines (sodium, thal- 

 lium, lithium, the blue line of strontium) have a centre of gravity 



