This is the 2610th most frequent Russian word.
тонкими
"Тонкими" – "thin" (instrumental plural form).
The word 'тонкими' is used in instrumental case plural form of the adjective 'тонкий', agreeing with 'линиями' to describe their thin nature.
Эти линии очень тонкими становятся при увеличении.
These lines become very thin when magnified.
The word 'тонкими' describes the slices of the bread, also in instrumental case plural, indicating their thinness as a characteristic.
Он нарезал хлеб тонкими ломтиками, чтобы всем хватило.
He sliced the bread into thin slices so that everyone would have enough.
The word 'тонкими' describes the brushes, showing their thickness in instrumental case plural, which allows for detailed work.