458 michelson's recent researches on light. 



tioTis were made witli a mnral circle and a repeating circle. Arago ex- 

 pected to find a difference of ten or fifteen 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 at- 

 tempted to explain his negative results by assumptions 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, 1810, and excited great interest. They 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 rei)utation 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 electri- 

 cal disturbance, or indeed any physical quantity which is capable o 

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

 connected with the particles of gross matter; otherwise they would 

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

 ajther by experiment, as the light must move forward and then back 

 again. One way is to compare the velocities of light obtained from the 

 eclipses of Jupiter's satellites when Jupiter is in opposite 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 sug- 

 gests a proper arrangement for this ])urpose. 



At various times between ]864 and 1868, Maxwell repeated Arago's 

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

 three prisms of 00° each. A plane miror was substituted for the slit of 

 the collimator. The cross- wires of the observing telescope were illumi- 

 nated 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 wires from which it 

 started after passing through six prisms. The experiment was tried 

 when the light started in the direction of the earth's motion, and when 

 in the opposite ; also, at different seasons of the year. In all cases the 

 image of the wires coalesced with the wires. 



Lodge states the case clearly thus : " If all the aether were free there 

 would have been a displacement of the image of the wires. If all the 

 a^'ther were bound to the glass there would have been a difference on 

 the other side. But, according to Fresnel's hypothesis there should be 

 no difference either way. According to his hypothesis, the free aether, 

 which is the portion in relative motion, has nothing to do with the re- 

 fraction. It is the addition of the bound »ther which causes the refrac- 

 tion, and this part is stationary relatively to the glass, and is not stream- 



