270 
DR, J. YOUNG AND PROFESSOR G. FORBES ON THE 
The value obtained by Michelson,* using a modification of the method of 
Foucault, was 299,940 kiloms. per second. He always used the light of the sun 
when near the horizon (in the early morning or late afternoon), except in a single 
set of observations where he used the electric light, and which he considered un¬ 
satisfactory. Thus we have three series of very carefully conducted experiments to 
determine the velocity of light, each one differing essentially from the others in their 
method of research ; the results are all very close to each other, and we believe that in 
the sequel we shall be able to show reasons for the outstanding differences. Grouping 
the three sets in order we have :— 
Michelson* . . . 
CoRNUf .... 
Young and Forbes . 
Usual source of light. 
Method. 
Results for V. 
The sun near horizon . 
Lime light .... 
Electric light 
Deflection by mirror .... 
Toothed wheel and eclipses 
Toothed wheel and equalities . 
Kiloms. 
299,940 
300,400 
301,382 
Cornu —Michelson. 460 kiloms. per second. 
Young and Forbes—Michelson. 1,442 „ „ 
Young and Forbes—Cornu . . 982 „ „ 
After we had completed the observations which have now been reduced, we found 
reason for believing that the velocity of light depends upon its colour, and further 
examination of the question confirmed us in this opinion. It seemed useless then to 
continue to measure the velocity of a light whose colour changes considerably and 
quite sufficiently to give us values for the velocity of light varying much more than 
any errors of observation could make them vary. We then devoted our attention to 
an examination of the question involved in the second part of our research, viz. : 
whether the velocity of light depends upon its colour, and if so to what degree. 
PART II. 
Relative Velocity of Lights of different Colours. 
Does the velocity of light de/pend upon its colour f 
Before describing the observations which furnish an answer to this question, we 
will briefly recapitulate the general arrangements in our method of working, so that 
those who wish it may study this part of our research independently of the rest. 
* “ Astronomical Papers prepared for the use of the American Ephemeris and Nautical Almanac.” 
Vol. i., part iii., 1880. 
| “ Annales de l’Ohservatoire de Paris ” (Memoires, tome xiii.), 1876, 
