Intelligence and Miscellaneous Articles. 551 



greater. From the number of bands from one Fraunhofer's line to 

 another, the wave-length of the latter may be calculated when that 

 of the first is known. 



To determine a wave-length directly, independent of another, the 

 difference of phase for the place of the spectrum in question must be 

 successively increased or diminished. Therefore a removal of the in« 

 terference-bands sets in. From the number of the bands which have 

 passed through the cross wires, and the change of the difference of 

 phase thus produced, the wave-length can be calculated for the place 

 fixed upon. The successive change of the difference of phase could be 

 obtained by pushing over each other two quartz wedges. Such an 

 apparatus was not at hand, and the following method was therefore 

 used. The column of quartz was turned slowly out of its position 

 at right angles to the incident rays ; the angle of incidence, and 

 therefore also the difference of phase between the ordinary and ex- 

 traordinary ray successively increased, and the bands which simulta- 

 neously passed through the cross wire were counted. As the change 

 in the difference of phase which occurs with the measured alteration 

 of the incident angle can be calculated, these are the data needed for 

 the absolute determination of the wave-length of the place fixed upon. 



For the wave-lengths of Fraunhofer's lines B, C, D, E, F, G, H, 

 the following numbers were found, in ten-millionths of a millimetre : 

 6873, 6578, 5893, 5271, 4869, 4291, 3959. These values agree 

 very accurately with those deduced from the diffraction phenomena 

 of fine gratings, and are therefore at the same time a proof of the 

 accuracy of our theory of the diffraction of light. — Berichte der 

 Wiener Akademie, April 26, 1866. 



ON THE INFLUENCE OF INTERNAL FRICTION TN THE AIR ON THE 

 MOTION OF SOUND. BY PROF. STEFAN. 



The results of the analytical investigation are as follows. Friction 

 increases the velocity of sound, and to a greater extent the higher 

 the tone. Yet even for the highest tones this increase is very small, 

 about 0*001 millim. in a second. 



The amplitudes decrease in plane-progressive waves in geometrical 

 progression. The exponent of this progression increases with the 

 height of the tone, and, indeed, proportionally to the square of the 

 number of vibrations. The diminution of amplitude is only percep- 

 tible in high tones. With a tone of 10,000 vibrations the amplitude 

 is diminished by ^ at a distance of 1000 metres ; at 2000 metres by 

 -£ T ; with a tone of 30,000 vibrations by -J, even at 100 metres. 



Standing vibrations are only possible if the length of the wave 

 exceeds a certain value. Yet this is very small, equal to four times 

 the mean way which, according to the new theory of gases, a mole- 

 cule makes from one impact to the next. 



In a standing wave also the amplitudes decrease with the time in 

 geometrical progression, whose exponent is proportional to the 

 square of the number of vibrations. The amplitudes of the tones of 

 1000, 10,000, and 30,000 vibrations sink to one-half before the lapse 

 of 100 seconds, 1, and 0*1 second respectively. — Berichte der Wiener 

 Akademie, April 16, 1866. 



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