390 PROCEEDINGS OF THE AMERICAN ACADEMY 



velocity of light would produce on aberration and refraction. He saw 

 that a change of five per cent in the velocity of light would alter the 

 aberration by only one second, whereas the refraction in a prism of 

 45° would be affected to the extent of two minutes. He observed the 

 zenith distances of stars with and without the prism ; and also the 

 deviation of stars which passed the meridian at 6 A. m. and 6 p. m. 

 The observations were made with a mural circle and a repeating cir- 

 cle. Arago expected to find a difference of 10 or 15 seconds, but 

 found none. He thought that a difference no greater than one ten- 

 thousandth would have been manifested by his observations had it 

 existed. Arago attempted to explain his negative results by assump- 

 tions based upon the corpuscular theory of light. But Lloyd thought 

 that the change in the length of the wave would balance the change in 

 the direction of the ray. Arago's observations were communicated to 

 the Institute on December 10, 1816, and excited great interest. Tliey 

 were quoted by Laplace and Biot. But the manuscript was mislaid 

 and not found until 1853, when it was published. Mascart thinks that 

 this experiment of Arago owes its reputation to Fresnel's explanation 

 of it by his fraction. 



In regard to the wave-motion involved in the transmission of light, 

 Maxwell says : " It may be a displacement, or a rotation, or an elec- 

 trical disturbance, or indeed any physical quantity wliich is capable of 

 assuming negative as well as positive values. But the ether is loosely 

 connected with the particles of gross matter: otherwise they would 

 reflect more light." Then he asks the question, " Does the ether 

 pass through bodies as water through the meshes of a net which is 

 towed by a boat ? " It is difficult to obtain the relative motion of the 

 earth and ether by experiment, as the light must move forward and 

 then back again. One way is to compare the velocities of light ob- 

 tained from the eclipses of Jupiter's satellites when Jupiter is in oppo- 

 site points of the ecliptic. Cornu referred, in 1883, to the difficulty 

 of observing these eclipses, especially when Jupiter is in conjunction 

 with the sun. On account of this difficulty observations have been 

 neglected for the last fifty years. Observations must be made near 

 quadratures. Cornu suggests a proper arrangement for this purpose. 



At various times between 1864 and 1868 Maxwell repeated Arago's 

 experiment in a more perfect form. A spectroscope was used, having 

 three prisms of 60° each. A plane mirror was substituted for the 

 slit of the collimator. The cross-wires of the observing telescope 

 were illuminated by light reflected by a plate of thin glass placed at 

 an angle of 45° . Light went to the mirror and was sent back to the 



