There, on what looked like a small television screen, flashes of light appeared that gradually formed a curve with many peaks and valleys. After a few minutes the spectrometer was stopped and an electric typewriter automatically typed out rows and columns of numbers. The chemist explained, “This curve, which you see on the screen, is a gamma-ray spectrum and tells us what elements are in the sample. The typed-out data give us an accurate measure of the shape of the curve on the screen. By measuring the gamma-rays’ energies we know what elements in the sample were made radioactive. The height of each gamma-ray peak tells us how much of that element is present in the sample. “That gives us the information we need to calculate the concentrations of the small quantities of materials in our samples. We can do this because at the same time I irradiated a set of standards. Standards are materials that are just like the samples except that they contain known amounts of the impurities I am trying to measure.” As the boys were leaving the laboratory, the chemist apologized for not having enough time to explain the activation analysis procedure more thoroughly, but he did give the boys a list of books to read on the subject of radioactivity and radioisotopes.[2] They thanked him for his help. During the ride home, they discussed the paintings that were still unproven. “It’s too bad that the method of activation analysis fingerprinting hasn’t been fully developed yet,” said Dad. “Yes,” said Bill. “Then we could prove whether or not that last old painting was really by Aelbert Cuyp as the expert from the gallery believed. But what about those paintings that we found in the box that were not so old?” “Well,” said Dad, “if the activation analysis method were workable, we might be able to prove if they were painted by Alfred Sisley. Meanwhile, until the method is really developed we don’t know if we can do it that way or not.” “So what do we do now?” asked Martin. “We’ll have to wait until scientists can thoroughly investigate this method and several others that they’re working on.” “Other methods!” exclaimed Bill. “What other methods?” [Illustration: _“The Banks of the Oise”, a genuine painting by Alfred Sisley._] Other New Tools for Art Authentication “There are several new tools that scientists are working on now,” said Dad. “These involve methods that have been developed by scientists for other purposes, but are now being explored for use in authenticating works of art. “For example, in Los Angeles, the county museum purchased an instrument known as a Spark Source Mass Spectrometer. Like activation analysis, this instrument will also measure small traces of impurities, but they have just set that up and it will take them years to explore the use of it for the type of problem we have been discussing.