1881.] Velocity of White and of Coloured Light. 247 



into small fragments with a sharp instrument before inserting it into 

 its new locus. 



6. In order to insure the success of the graft the transplantation 

 must be conducted antiseptically. 



V. " Experimental Determination of the Velocity of White and 

 of Coloured Light." . By Dr. J. Young, F.R.S., and Pro- 

 fessor G. Forbes. Received May 17, 1881. 



(Abstract.) 



The method employed in this research to measure the velocity of 

 light resembled the method of M. Fizeau, subsequently employed by 

 M. Cornn. A revolving toothed wheel is employed in the same way 

 to alter the intensity of the light reflected from a distance. In the 

 present method, however, there are two distant reflectors instead of 

 only one. They are separated by a distance of a quarter of a mile. 

 The observing telescope and the two reflectors are almost in the same 

 line. The observer sees two stars of light which go through their 

 phases with different periods as the toothed wheel is revolved at in- 

 creasing speeds. One star is increasing, while the other is diminish- 

 ing, in intensity, with increase of speed of the toothed wheel. The 

 speed required to produce equality of the lights is determined by 

 means of a chronograph. By choosing such a speed as gives a maxi- 

 mum of one star at the same speed as a minimum of the other, a pair 

 of observations eliminates all cause of doubt arising from varying 

 brightness in the stars, and ratio of the width of a tooth to the width 

 of a space. The distances were observed by triangulation with the Ord- 

 nance Survey 18-inch theodolite, using as a base line a side of one 

 of the Ordnance Survey triangles. The source of light was an electric 

 lamp. The velocities (uncorrected for rate of clock, and reduction to 

 & vacuum) measured are as follows : — 



187,707 



188,405 



187,676 



186,457 



185,788 



186,495 



187,003 



186,190 



186,830 



187,266 



188,110 



188,079 



Mean 187,167 miles a second. 



vol. xxxn. s 



