The agreement became still better when the residual rays of fluorite with a wave-length of 25·5 μ were used. Since, however, for waves of this length the reflexion becomes nearly complete, it was not possible to determine the loss of energy with sufficient precision. Hagen and Rubens overcame this difficulty by measuring the emissivity of the different metals, or rather the ratio between this emissivity and that of a perfectly black body at the same temperature, a ratio which, by Kirchhoff's law, is equal to that between the absorbed and the incident energy for a beam falling on the metal from the outside, so that it can be calculated by the same formula as this latter ratio. For the four metals just mentioned (at a temperature of 170° C.) the ratio in question was found to be (after multiplication by 100) 1·13, 1·17, 1·56 and 2·82. The theoretical values were 1·15, 1·29, 1·39 and 2·96. These numbers show conclusively that, however complicated things may be for shorter waves, we can calculate the optical properties of metals in the extreme infra-red by means of Maxwell's equations, simply substituting for the conductivity the value that has been deduced from experiments with constant or slowly alternating currents. This is certainly a most splendid confirmation, the counterpart to the verification, which for gaseous bodies at least has been very satisfactory, of Maxwell's relation of the dielectric constant to the index of refraction. The phenomenon of the pressure of radiation may serve as a second example of verification. That a beam of light falling, say in the normal direction, on a mirror exerts on it a pressure proportional to the intensity of the beam was deduced by Maxwell from his formulae, and he calculated the force that may be expected in the case of sunlight. It lasted a quarter of a century before Lebedew succeeded in observing this small force, which, for sunlight, amounts to no more than about a ten millionth part of a gramme weight per cm.^2, and which it is therefore difficult to disentangle from other forces that are caused by the surrounding gas, even when this is highly rarefied. Some years later E. F. Nichols and Hull repeated the experiment with the utmost care and were able to measure the pressure and to prove that its intensity agrees with Maxwell's calculation. We are now quite sure of this phenomenon which has come to play a great part in stellar physics. When we are concerned with very small particles near or in a star, the radiation pressure may very well become greater than the force of gravitation, and it is taken into account by many astronomers in their speculations about the state of heavenly bodies.