"I never thought that one could grow so attached to science." These words are from a letter that the young Russian physicist Pyotr Lebedev wrote to his mother from Strassburg,  where he had gone to complete his education.  

           Lebedev was born in Moscow on March 8, 1866, he was the son of a wealthy merchant. His father wanted him to follow a business career and sent him to the German Commercial School in Moscow. In those days many of the schools were run by foreigners, and it was the fashion among merchants to educate their children at them.

        Early in life, however, Lebedev was attracted by science. Gradually he became convinced that his calling was research. Training for research work could only be had at the universities,   but to enter one of those it was essential to know Latin and Greek, which Lebedev had never studied. He was therefore compelled to go abroad for a scientific education.

      In Strassburg Lebedev made a thorough study of physics and became very good at experiments. In 1891 he received his doctor's degree, after completing a brilliant dissertation on the dielectric permeability of gases. These studies convinced him that an electric field exerts a strong influence on the properties of molecules, a subject that continued to hold his interest.

       The study of this problem led Lebedev to an impor­tant discovery, which concerned the tails of comets. Down the ages people have watched the movement of comets with wonder. Gradually comets came to be accepted as a part of the Universe, just like the stars and planets. On the basis of physical laws, scientists determined the orbits in which comets move in space and even learned to predict the dates of their periodic appearance. It was established that a comet consists of a small "head", made up of extremely dense matter, and a "tail", a huge amount of rarified matter.

     However, scientists could not explain one exceed­ingly interesting circumstance. As it approaches the Sun, a comet changes its shape in a very definite manner. Its tail lengthens out at a rate of a million kilometres a day until it reaches a maximum length of 50—100 million kilometres at the point nearest to the Sun. Then, as it withdraws from the Sun, its tail shortens in a similar manner. The most remarkable thing, however, is that the tail always points away from the Sun. This fact appeared to contradict the law of gravity, according to which the tail of a comet, like any other material body, should be attracted to the Sun.

In fact, the opposite seems to be true: the tail appears to be repelled by the Sun.  

      As early as 1619 Johannes Kepler had suggested that this behaviour of a comet's tail was due to the repulsive action of the Sun's rays; but nobody could provide a plausible explanation of the mechanism of this repulsion.  In 1891 Lebedev wrote a paper entitled "On the Repulsive Force of Radiating Bodies", in which he offered a theory explaining the changes in the shape of a comet's tail. In his paper he demonstrated that there are forces of repulsion in space, which are brought into being by the pressure of light.   

      Moreover, for very small cosmic particles the force of repulsion due to the pressure of light is considerably greater than the force of attraction due to the law of gravity. Comets' tails, Lebedev argued, consist of gases, which are actually a vast quantity of molecules. As the comet draws nearer to the Sun, each of these molecules ex­periences an increasing repulsive force of the Sun's rays. It is the light rays that drive the particles of the tail away from the Sun.

      Lebedev next set out to prove in laboratory conditions that light exerts pressure. Some years earlier the English physicist James Clerk Maxwell had formulated an electromagnetic theory of light, in which it was assumed that light does exert pressure. But Maxwell's theory was purely mathematical. There was no ex­perimental confirmation of it, and many very prominent scientists were opposed to it.

    If the pressure of light could be established ex­perimentally, this would clearly prove Maxwell's theory correct.  But to detect this pressure was a very difficult matter: a mosquito alighting on a mirror will press on it with a greater force than a ray of sunlight!   Nevertheless, by means of an extremely clever experiment, Lebedev succeeded in actually measuring the pressure of light. Lebedev first reported his experiment at a  meeting of a Swiss Scientific Society in 1899. The following year he reported the results of his work to the World Congress of Physicists in Paris.

     The discovery made a profound impression.' The Russian scientist's success was applauded by all the leading physicists of the world. The great Lord Kelvin, on meeting Kliment Timiryazev, said to him: "Perhaps you know that all my life I have been fighting Max­well, refusing to recognize his light pressure, and now your Lebedev has forced me to surrender   in the face of his experiments!"