LIGHT. 



35 



the solution, I the length of the solution in decimetres, and w the 

 mass of the active substance. ( + ) right-handed and ( - ) left-handed 

 rotation. The rotation required to reproduce the sensitive tints. t, 

 which is the peculiar grey given when the yellow is absorbed from 

 white light, is equal to that of the mean ye 1 low. 



Rock crystal 1 mm. 

 D 



Kock crystal, sensi- 

 tive tint 



Cinnabar, 2 rnm....B 



Strychnine sulphate 

 -f 13 aq. 1 mm. B 



Sodium chlorate 2'25 

 mm 



Potassium thiosul- 

 phite, 1 mm 



Cane sugar s.t. 



Levulose s.t. 



Glucose .s.t. 



Milk sugar s.t. 



Mannite D 



Camphor in alcohol 

 s.t. 



Dextrin D 



Turpentine D 



Tartaric acid D 



Ammonium tartrate 

 D 



Egg albumin D 



Amyl alcohol D 



Quinine sulphate (red) 

 Strychnine (red) 



+ 59 



- 0-15 



+ 47*4 



+ 138-7 



- 43-5 

 9-6 



+ 29 



- 35-5 



- 4-88 



- 147-7 



- 132 



(44) THE VELOCITY OF LIGHT IN METMES PER SECOND. 



Romer (1676), eclipses of Jupiter's satellites ... 310000000 



By aberration of the fixed stars (20 '445") 308 300 000 



Fizeau, (telescopes and toothed wheel) 315 000 000 



Foucault (1862), (revolving mirror in air) 298 000 000 



Cornu (1873) in air 298 400 000 



,, (1873) in vacuo 298500000 



,, (1874) in air 299740000 



,, (lS74)wraco 300400000 



Michelson (1879) in air , 299 740 000 



(1879) in vacua 299820000 



Hence the velocity of light in vacuo most probably is 3'004 x I0 5 

 kilom. or 186000 miles per second. The denser the medium 

 through which the light is passing the less is the velocity. If fj. be 

 the absolute refractive index of light of a given refrangibility in any 



. 300400000 

 medium, the velocity is - metres per second. 



