90 



SCIENCE. 



[N. S. Vol. XVI. No. 394. 



ent colors this method would be capable of 

 showing even a very slight difference. For 

 example the difference in velocity between 

 the extreme red and the violet rays in 

 carbon disulpliide is about one sixteenth. 

 For air this difference is about one part 

 in one hundred thousand. Now an addi- 

 tion of five per cent, of one of these colors 

 and a subtraction of the same amount of 

 the other from white light will produce a 

 perceptible change in the time. Thus if 

 we consider a five per cent, change instead 

 of one per cent, as mentioned previously, 

 and multiply this by sixteen we obtain 

 1.6 meters as the length of a column of car- 

 bon disulphide between the grating and the 

 fixed mirror, in which we could just detect 

 the dispersion of light. Similarly in the 

 case of air we have to multiply by five and 

 by 100,000 and obtain about ten kilometers 

 to produce the necessary dispersion in air. 

 It is very doubtful whether this sensibility 

 can be actually realized. In the prelimin- 

 ary experiments with his instrument sun- 

 light was used. The scattered light from 

 different parts of the system prevented the 

 contrast between the light and dark spaces 

 in the return image which would be nec- 

 essary for such a high sensibility. By ad- 

 justing the mirror 31 so that the images 

 of the bright spaces of the grating should 

 fall on the dark spaces on their return, 

 the appearance for an eclipse could be 

 studied. In reference to the intensity of 

 the return image, if we allow fifty per cent. 

 less from successive reflections and remem- 

 ber that the angle of the grating was 

 only one sixtieth of the circumference, 

 we find only about one per cent, of the 

 light available in the return image. Thus 

 with ,the mechanical system of Fizeau's 

 toothed wheel we should have one hundred 

 times the light available for the same aper- 

 ture. The preliminary experiments were 

 made under unfavorable conditions, but 



indicated a greater sensibility than that 

 heretofore obtained. With sunlight suffi- 

 cient intensity would be available for the 

 experiments just i-eferred to. With the 

 apparatus as mounted too much instability 

 was experienced to obtain satisfactory defi- 

 nition with the rotating mirror and the 

 motor and blower which drove it in mo- 

 tion. The condition of the faces of the 

 rotating mirror also prevented distinct 

 definition. In order to use all four faces 

 they required refiguring and the angles 

 between them recut. The lack of funds 

 to make these changes and the recall of the 

 instrument have caused the experiments 

 to be interrupted several times and finally 

 abandoned until the means for building 

 apparatus more suitable for the purpose 

 and for carrying out the experiment can 

 be obtained. This method will give the 

 group-velocity V directly according to the 

 criticism already referred to on the method 

 of the rotating-mirror. The possibility of 

 obtaining important data on group-velocity 

 in different media by so sensitive a method, 

 and also the determination of the velocity 

 of light in a vacuum itself warrants fur- 

 ther eft'orts being made along this line. 

 These experiments were initiated in 1889 

 and discontinued a half dozen years later 

 in the way mentioned. 



It is doubtful whether there are other 

 methods for determining the group-veloc- 

 ity of greater sensibility. One method of 

 considerable promise is by means of polar- 

 ized light. Two rays of light pass through 

 a Nichol and a half -wave plate. Each is re- 

 flected from each of two mirrors respect- 

 ively. By properly focusing we should 

 have a half-shade combination which on 

 rotating the half -wave plate and the Nichol 

 about the common axis would give a dif- 

 ference in the intensity of each return ray. 

 Half-shade systems have a sensibility as 

 high as one thousandth of a degree in the 



