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CHAPTER VIII.

LIGHT.

85. ALL kinds of light are believed to have the same velocity in vacuo. The velocity of light of given refrangibility in any medium is

1

of its velocity in vacuo,

μ denoting the absolute index of refraction of that medium for light of the given refrangibility.

Light of given refrangibility is light of given wavefrequency. Its wave-length in any medium is the quotient of its velocity in that medium by its wavefrequency. If n denote the wave-frequency (that is to say, the number of waves which traverse a given point 1 in a second), the wave-length in any medium will be

of the velocity in vacuo.

ημ

The absolute index of refraction for ordinary air is about 1.00029. More accurate statements of its value will be found in Arts. 94-96.

86. The best determination of the velocity of light is that made by Professor Newcomb at Washington in 1882 ("Astron. Papers of Amer. Ephem.," vol. ii. parts iii. and iv. 1885). The method employed was that of the revolving mirror, the distance between the revolving and

the fixed mirror being in one portion of the observations 2550 metres, and in the remaining portion 3720 metres. The resulting velocity in vacuo is

2.99860 × 1010 centims. per sec.

The following summary of results is from Professor Newcomb's paper, page 202 :

:

km. per. sec.

Professor Newcomb remarks (page 203) that the value 299860 km. per sec. for the velocity of light, combined with Clark's value 6378.2 km. for the earth's equatorial radius, and Nyren's value 20"-492 for the constant of aberration, gives for the solar parallax the value 8" 794.

87. The following are the wave-lengths adopted by Ångström for the principal Fraunhofer lines in air at 760 millims. pressure (at Upsal) and 16° C. :

Centims.

These numbers will be approximately converted into the corresponding wave-lengths in vacuo by multiplying them by 1.00029.

88. Assuming 3 x 1010 to be the velocity of light in air, and neglecting the difference of velocity between the more and less refrangible rays, we obtain the following frequencies by dividing the common velocity by Ångström's values of the wave-lengths :—

According to Langley ("Com. Ren.," Jan., 1886), the solar spectrum extends beyond the red as far as wavelength 27 × 10-5, and the radiation from terrestrial bodies at temperatures below 100° extends as far as wave-length 150 x 10-5. The frequencies corresponding to these two wave-lengths are 1·1 x 1014 and 2 × 1013.

INDICES OF REFRACTION OF SOLIDS.

89. Dr. Hopkinson (" Proc. R. S.," June 14, 1877) has determined the indices of refraction of the principal varieties of optical glass made by Messrs. Chance, for the fixed lines A, B, C, D, E, b, F, (G), G, h, H1. By D is to be understood the more refrangible of the pair of sodium lines; by b the most refrangible of the group of magnesium lines; by (G) the hydrogen line near G.

In connection with the results of observation, he

employs the empirical formula

μ-1=a{1 + bx(1 + cx)},

where x is a numerical name for the definite ray of which μ is the refractive index. In assigning the value of x, four glasses-hard crown, soft crown, light flint, and dense flint were selected on account of the good accord of their results; and the mean of their indices for any given ray being denoted by μ, the value assigned to x for this ray is μ-μr where μr denotes the value of μ for the line F.

The value of as a function of A, the wave-length in 10 centimetres, was found to be approximately

The following were the results obtained for the different

specimens of glass examined :—

Hard Crown, 1st specimen, density 2.48575.

a=0·523145, b=1·3077, c=- -2.33.

Means of observed values of μ.

A 1.511755;

B 1.513624; C 1·514571;

E 1.520324;

G 1.528348;

D 1.517116;

b 1.520962; F 1523145; (G) 1·527996; h 1.530904; H1 1.532789.

Soft Crown, density 2.55035. a=0·5209904, b=1·4034, c== -1.58. Means of observed values of μ. B 1.510918; C 1.511910; b 1.518678; F 1.520994; (G) 1.526208; G 1.526592; h 1.529360; H1 1.531415.

A 1.508956;

E 1.518017;

D 1.514580;