THINGS HAVE changed since the days when James Whistler did his famous portrait of his mother in 1871. If he were painting it today, this is how the conversation might go:

"Okay, Mom, just sit in this straight chair over here. I'm going to make you famous."

"How come I only see you when you want to paint me?"

"Aw, come on, Mom, don't start that again. I've been busy at the advertising agency. I call you once a week. Isn't that enough?"

"Mrs. Phillips' son comes over to the house every Sunday for lunch."

"You know I play tennis on Sunday at the club. It's the only chance I have to get some exercise. Please, Mom, I don't have much time."

"Don't you think I ought to put on a nice dress if you're going to paint me?"

"The one you're wearing is perfect. Here put on this lace hat, and face the window. I want to do you in profile."

"Are you ashamed to do me full face?"

"No, but I want something different. This is for the 'Mother McGarry's Homemade Pie' campaign. It will appear in magazines and newspapers all over the country."

"Why don't you paint Mother McGarry?"

"There is no Mother McGarry. The company's owned by a conglomerate out of St. Paul. Okay, Mom, hands on the lap, and feet on the stool."

"I thought you were going to paint me. What are you doing with a camera?"

"I'm taking Polaroid pictures, because I can only give you one sitting. I'll work from the photos at my office."

"You mean this will be the only time I get to see you?"

"Mom, just look at the window and don't talk. I want to get a beautiful, motherly expression on your face. Don't smile, for heaven's sakes!"

"Why shouldn't I smile?"

"Because Mother McGarry is supposed to be a serious person. That's why her homemade pies taste so good. Think of something sad. That's it. What are you thinking about?"

"My Social Security Check, and how it all went for oil last month."

"I'm sending you $50 a month."

"Have you ever tried to eat on $50 a month?"

"I'd make it more if I could, Mom. But this inflation is killing me. Okay, I think I have enough photos. Now I'll start sketching. Remain absolutely quiet."

"My back hurts in this chair. Couldn't I have a cushion?"

"It will ruin the painting. Mother McGarry has to look like she's been baking all day long. Now remember, you're tired, but, at the same time, happy that every pie turned out just as delicious as the first one."

"If you stay for dinner, I'll bake you a real pie."

"I can't. We're going to a cocktail party tonight at the Wellingtons'. That's it, hold that expression. You know, they wanted me to get a professional model, but I said my mother would sit for free."

"I sacrificed and sent you to art school so someday you'd be doing paintings for phony homemade pies?"

"Mom, I have to make a living. Artists starve to death. It's worse for portrait painters. If I tried to sell this picture to a museum they'd laugh at me. The money's in advertising."

"My back's killing me."

"It will only take a few more minutes. If this picture doesn't sell pies, nothing will."

"So when will I see you again?"

"It all depends. If I can work from the Polaroid pictures, I won't have to come back. But you may have to come up to the office in case they want me to paint a pie on your lap."