310 Intelligence and Miscellaneous Articles. 



the most marked, the others having been extinguished by the effect 

 of distance ? " 



Moreover, if it were proved that the other lines of the spectra 

 of nitrogen and hydrogen are stopped in their passage, we might 

 regard this as an indication of the extinctive power of cosmic spaces. 

 For this reason we have endeavoured, in imitation of Huggins, to 

 ascertain by experiment if an alteration in the luminous intensity 

 of a gas, iviihout modification of the temperature and pressure of the 

 gas, would correspond with the disappearance of one or more lines 

 in the spectrum. 



The method we have employed consists in projecting, by means 

 of a lens, a real image of the luminous body upon the slit of 

 the spectroscope, and then altering the intensity of that image, 

 either by reducing the aperture of the projecting lens, or by shift- 

 ing a diaphragm pierced with a circular aperture between the lens 

 and the projected image. All the observations took place in a dark 

 room, the apparatus being arranged as follows : — 



(1) A Melloni optical bench supporting the projection-objective 

 and the diaphragm. The objective has a clear opening of 8 centims. ; 

 the diaphragm is that of Melloni's apparatus. 



(2) An automatic reversing-spectroscope of Toung. with a vari- 

 able dispersion of from two to ten prisms, placed behind the optical 

 bench. 



The observing-telescope is of the same focal length, parallel, and 

 twinned with the collimator. The two objectives are thus shifted 

 simultaneously, and to the same extent, at the time of adjusting. 

 The slit of the spectroscope is horizontal ; and the optic axis of the 

 collimator coincides with that of the projection-objective. 



(3) A Pliicker's tube coated all over with lampblack, with the 

 exception of a small portion of the straight part, is placed vertically 

 (upon a movable foot, permitting it to be fixed at a suitable height) 

 in front of the projection-objective. 



The distances from the tube to this objective, and from this to 

 the slit, are so combined that the width of the projected image is 

 less than the length of the slit, and that the whole of the luminous 

 pencil passing through the slit is received upon the objective of the 

 collimator. This last arrangement, devised by Lockyer, constitutes 

 a most important advance in the improvement of the methods of 

 spectral observation : it permits us to recognize in a spectrum the 

 long lines and the short lines, which were first discovered by that 

 accomplished English physicist. 



In our experiments the Pliicker tube was placed at 40 centims. 

 from the front lens of the objective, and about 1*3 metre from the 

 slit of the spectroscope. 



The induction-current was furnished by a Gaiffe induction-coil of 

 large dimensions, set in action by a battery of eight couples with 

 bichromate of potass, and capable of giving a spark of 50 centims. 

 in free air. The intensity of the current was regulated by the 

 greater or less immersion of the polar plates in the acid solution. 



A condenser, formed of ten fulminating squares having a total 

 surface of 5 square metres, and arranged either in tension or in 



