What exercise would you recommend for the training of the fourth and fifth fingers of the left hand?

Slow trill with various touches, with highly lifted fingers producing strength through their fall and with a lesser lift of the fingers combined with pressure touch, watching closely that the little finger strikes with the tip and not with the side. Rhythmic evenness should also be punctiliously observed.

When the Fingers Seem Weak

What kind of technical work would you advise me to take to make my fingers strong in the shortest time consistent with good work?

If your fingers are unusually weak it may be assumed that your muscular constitution in general is not strong. The training of the fingers alone will, in that case, lead to no decisive results. You will have to strive for a general strengthening of your muscular fibre. At this point, however, begins the province of your physician and mine ends. If you consider your constitution normal, four or five hours' daily work at the piano will develop the necessary digital force, if that time is judiciously used.

No Necessity to Watch the Fingers

Is it always necessary to watch the fingers with the eye?

In places where the fingers slide, and do not jump from one note to another at a distance, there is no need of keeping the eye on them.

Biting the Finger-Nails Spoils the Touch

Is biting the finger-nails injurious to the piano touch?