The shoemaker's ball

Your assumption is wrong.

Even though the shoemaker's ball focusses the rays of light onto a small area, the fingers could be warmed up more easily. You could warm up your hands well around a fire or with a warm cup of tea. But surely the warmth would often be a welcome side effect of the shoemaker's ball.

Indeed, the shoemaker's ball was used to work better with the natural light. Before electric lighting was invented, it was very difficult to light rooms selectively and uniformly. If, in some cases, candles and gas lamps were not useful, people tried to utilise the available natural daylight in the best way possible.

But in what way was the shoemaker's ball used exaclty?

a) The ball was used in the morning to distribute the sunlight which was still shining selectively. The whole room was illuminated by the shoemaker's ball.

b) People used the shoemaker's ball at lunchtime to deflect the bright, dazzling light of the sun.

c) The shoemaker's ball was especially used in the evening to focus the light, which was still available during dusk, for work.