You already know his chair. It is the one that faces the window, the one with the dent in the cushion and the throw folded over the arm. The lamp beside it may be the lamp she used for her crossword, standing exactly where she needed it, and it may be the wrong height for the man who sits there now.
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Sit in His Chair Before You Move Anything
Sit where he sits, at the time he usually reads, with the overhead light off. Hold a book or the newspaper the way he holds it. Check whether the light lands on the page or on your hands, and whether the bulb shines straight into your eyes. If you are squinting, he probably is too.
Measure three things while you are there: the distance from the seat to the lamp base, the height of the shade bottom above the armrest, and the distance from the outlet to the side of the chair. Write the numbers down. You will need them whether you keep the old lamp or buy a new one.
Match the Shade, the Bulb, and the Switch
The bottom edge of the shade should sit a little below his eye line when he is seated, so the bulb is hidden and the light falls down onto the page. A shade that is too tall throws light onto the ceiling and leaves the book dim. A shade that is too low puts the bulb in his face.
Check the wattage printed on the base or the socket and use only what it allows. Pick a bulb by how bright the page looks, not by the number on the box. Keep a spare in the drawer nearest the chair so the lamp is never dark for a week while someone goes looking for one.
Check the switch. He should be able to turn the lamp on without leaning forward or standing up. A switch on the cord at hand height works well. A lamp that needs a twist at the base may be fine for you and frustrating for him every evening.
Look at the cord. It should reach from the outlet to the chair with a little slack, and it should not cross the path between the chair and the kitchen. A cord that runs along the floor is a trip waiting for a slipper.
What to Do Tonight
Sit in his chair with the overhead light off and see whether you can read the page on the table. If you cannot, move the lamp closer or swap the bulb. Do not throw the old lamp away yet. Put it in a closet and ask him in a week how the new light feels. Ask plainly and give him an easy way out of the answer.
When to Ask the Doctor
If his eyes tire quickly, if he holds reading material very close, or if he has stopped reading things he used to enjoy, mention it at his next visit. A lamp change may help, but his eyes may need a checkup of their own. Tell the doctor exactly what you have seen rather than saying he seems off.