Listen again to part of the lecture, then answer the question. Why would the professor say this?

Listen to part of a lecture in an art history class.
Professor: In our last lecture, we explored indirect influences that painters had on later generations of painters. Let’s now turn our attention to a painter who was famous not just for his paintings, but for his influence, his direct influence on other painters during his own time. You might have guessed that I’m talking about Rembrandt.
    Rembrandt taught just about every significant painter of his era. We’re talking about the Netherlands in the 17th Century. Rembrandt was so influential that there have been lingering questions about the authenticity of some of the paintings attributed to him. This is because he ran workshops where he actively encouraged his pupils to imitate his style. He even collaborated with them on some pieces. So since the late 1960s there have been serious efforts to examine his paintings to prove that it was indeed Rembrandt himself who painted them. Well, what happened was, the really interesting results of these investigations had to do with not so much authentication, but what the results revealed about Rembrandt’s methods, his procedures.
    Now, one aspect of these paintings that the investigators focused on was the lower layers of paint. The first layers of paint were applied to canvas or wood panel. You have a couple of ways of seeing under the surface. Conventionally, we would use X-ray machine. X-radiography is the same technique a doctor uses to take a picture of your bones, but if you X-ray a painting, well, you can see the lower layers all right, but only the lighter areas. The dark areas don’t show up, but there’s another technique that does reveal the darker shades. It’s a little more complicated. This technique is called "autoradiography." It involves exposing the painting to mild radiation. As different colors use different pigments, the different pigments will have different reactions to the radiation. Some will hold onto the radiation longer than others. That is, the rate of decay is not the same for all pigments. So, if you were to lay a strip of X-ray film over the painting, say five minutes after it was exposed to the radiation, it would pick up one pigment. Lay a strip over the painting after a month and it would reveal an entirely different pigment.
    Ok, so now with autoradiography researchers could see the darker shades underneath as well as the lighter areas, and it allowed them to reconstruct how the paintings were started, and what they were able to see was the importance of something called the "under-painting." Now, under-painting... ok, under-painting looks like the finished painting, but it was completely done in shades of just one color, usually brown or sometimes gray. So with under-painting an artist was better able to envision his final painting. This confirmed report from his students and from others that Rembrandt didn’t work from paper sketches. Basically, he had the whole picture worked out in his mind before ever setting brush to canvas. Another thing the autoradiography confirmed is that Rembrandt didn’t work with a full pallet. What I mean is he’d divide the painting up into sections, and work only with those colors he needed for a particular section before moving on to the next. This is quite different from how later painters would work, A 19th century French artist, like Cezanne for instance... Cezanne would work on the painting as a whole, using a full pallet of colors. It looks like painters of Rembrandt’s era only used a full pallet at the very end when touching up and almost finished painting.
    So we ended up learning more from radiography about Rembrandt’s techniques than about how to authenticate his paintings, but it was an amazing project. I mean the researchers were very lucky to have access to these paintings at all. Generally, you can’t just take paintings off a museum wall and start analyzing them, especially when this involves removing the paintings from public view for months on end. I’m afraid in the future we’re going to have to find different methods for analyzing other works.