[ 317 ] 

 XL. Intelligence and Miscellaneous Articles. 



ON THE PHOTOMETRY OF COLOURED FLAMES. BY M. GOUY. 

 HPHE photometric process of which the description forms the 

 ■*- subject of this Note permits us to measure with facility the 

 brightness of the various lines constituting the spectrum of co- 

 loured flames. 



"When a source of light is examined with the spectroscope, a 

 spectrum is seen of which the brightness varies according to the 

 point considered, the nature of the source, and the width of the 

 slit. The luminous intensity in a point of the spectrum is ex- 



pressed by the integral h ! id X. 



In this formula, i is the intensity of the ray whose wave-length 

 is \, emitted by the source ; I' is a coefficient depending on the 

 construction of the apparatus and the loss of light by absorption 

 and reflection ; \ l and A 2 are two wave-lengths such that a source 

 which emitted only those two rays would give in the spectroscope 

 two Hues in contact whose common margin would pass through 

 the point considered. 



If we project upon the slit of the spectroscope, supposed verti- 

 cal, the image of a Babinet's compensator arranged so as to give 

 horizontal fringes, illuminating it by two luminous pencils polarized 

 at a right angle and emitted by two different sources, a spectrum 

 will be produced streaked with horizontal fringes. Each of the 

 sources will give a system of fringes ; and the dark fringes of the 

 one system being superposed to the bright ones of the other, the 

 portions of the spectrum in which the preceding integral has the 

 same value for both sources will alone preserve their usual aspect. 

 By varying in a known ratio the intensity of the light emitted by 

 one of the sources, we shall be able to measure the brightness of 

 the different parts of the spectrum which it produces. If it gives 

 a line- spectrum, the slit is to be opened sufficiently for the lines 

 to become broad bands ; this will make the observation easier. 



The apparatus is arranged thus : — In front of the slit of a 

 spectroscope is placed the photometer, which, comprises the follow- 

 ing pieces, ranged in a straight line iu the axis of the collimator: — ■ 

 an achromatic lens ; a Nicol with its principal section horizontal ; 

 a piece similar to a Babinet compensator, in which the axes of the 

 quartz prisms are at 45° from the fringes, which are horizontal ; 

 an achromatized spar prism, of wdiich the principal section is 

 horizontal ; a Nicol movable at the centre of a graduated circle ; 

 and a lens. 



Between the spar prism and the last Xicol the tube carries a 

 lateral branch at a right angle, which contains a total-reflection 

 prism and a lens. A flame being placed before this lateral branch, 

 the light which is not slopped by the diaphragms passes through 

 the spar as the extraordinary ray ; the contrary takes place for a 

 flame placed before the movable Nicol. The two flames thus give 

 complementary fringes, the image of which is thrown upon the slit 

 of the spectroscope and passes into the spectrum. 



