INDUSTRIAL PROGRESS DURING THE YEAR 1875. lxi 



heated in the exterior combustion zone to cause their de- 

 composition. 



Gariel has described some simple apparatus for explaining 

 by construction the elementary laws and formulas of optics. 



Cornu has communicated to the Academy a valuable pa- 

 per on the velocity of light, in which he gives the results of 

 the new measurements made between the Paris Observatory 

 and the tower of Montlhery, twenty-three kilometers distant, 

 under the direction of the council of the observatory. As a 

 mean of 504 experiments, he finds the velocity of light in 

 vacuo to be 300,400 kilometers, or 186,700 English miles, 

 w T ith a probable error below one thousandth in relative value. 

 This gives for the solar parallax, as found by the equation 

 of light, 8.878", and by the phenomena of aberration, 8.881". 



The same author has described a new measuring instrument 

 for minute quantities, called a reflection lever, which consists 

 of a beam like a balance beam standing on four points, two 

 on the line where the knife edge is usually placed, the other 

 two at the ends of the beam, all four being accurately in one 

 plane. To the centre of the beam is attached transversely 

 a mirror, by means of which any displacement from the hor- 

 izontal may be detected and measured by the reflected lin- 

 age of a distant scale. The readings are made with a tele- 

 scope. 



Pickering and Strange have investigated photometrically 

 the amount of light absorbed by the sun's atmosphere. By 

 means of a porte lumiere carrying a black mirror and lens, 

 an image of the sun 40 centimeters in diameter was thrown 

 on a screen 230 centimeters from the aperture. A circular 

 hole was cut in the screen, and behind this the photometer 

 disk was placed. By moving the mirror any portion of the 

 sun's image could be thrown on the photometer, and its 

 light measured. The results were thus given : The proba- 

 ble error does not exceed one per cent., except close to the 

 edge. The light at the edge is about 0.4 that at the centre. 

 The variations in brightness are nearly those which would 

 be produced by a homogeneous atmosphere whose height is 

 equal to the sun's radius, and its opacity such that only twen- 

 ty-six per cent, of the light is transmitted. There appears 

 to be a slightly different distribution of the light along the 

 polar from that along the equatorial diameter. If the sun's 



