200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
to still regard them as forming one unit. Thus the energy in the 
bundles of light, after they have suffered partial reflection, will not 
be the same as in the bundles when they were emitted. The study 
of the dimensions of these bundles, for example, the angle they sub- 
tend at the luminous source, is an interesting subject for investiga- 
tion; experiments on interference between rays of light emerging in 
different directions from the luminous source would probably throw 
light on this point. 
I now pass to a very brief consideration of one of the most impor- 
tant and interesting advances ever made in physics, and in which 
Canada, as the place of the labors of Professors Rutherford and 
Soddy, has taken a conspicuous part. I mean the discovery and in- 
vestigation of radioactivity. Radioactivity was brought to light by 
the Réntgen rays. One of the many remarkable properties of these 
rays is to excite phosphorescence in certain substances, including the 
_ salts of uranium, when they fall upon them. Since Réntgen rays 
produce phosphorescence, it occurred to Becquerel to try whether 
phosphoresence would produce Réntgen rays. He took some uranium 
salts which had been made to phosphoresce by exposure not to 
Réntgen rays but to sunlight, tested them, and found that they gave 
out rays possessing properties similar to Réntgen rays. Further in- 
vestigation showed, however, that to get these rays it was not neces- 
sary to make the uranium phosphoresce, that the salts were just as 
active if they had been kept in the dark. It thus appeared that the 
property was due to the metal and not to the phosphorescence, and 
that uranium and its compounds possessed the power of giving out 
rays which, like Rontgen rays, affect a photographic plate, make 
certain minerals phosphoresce, and make gases through which they 
pass conductors of electricity. 
Niepce de Saint-Victor had observed some years before this dis- 
covery that paper soaked in a solution of uranium nitrate affected a 
photographic plate, but the observation excited but little interest. 
The ground had not been prepared, by the discovery of the Réntgen 
rays, for its reception, and it withered and was soon forgotten. 
Shortly after Becquerel’s discovery of uranium, Schmidt found 
that thorium possessed similar properties. Then Monsieur and 
Madame Curie, after a most difficult and laborious investigation, dis- 
covered two new substances, radium amd polonium, possessing this 
property to an enormously greater extent than either thorium or 
uranium, and this was followed by the discovery of actinium by 
Debierne. Now the researches of Rutherford and others have led to 
the discovery of so many new radioactive substances that any attempt 
at christening seems to have been abandoned, and they are denoted, 
like policemen, by the letters of the alphabet. 
