The instrument prevents the singer's listening to the tone he is producing and judging of its effectiveness.
Pupils with high or very low voices may continue their practice higher or lower as the voice is soprano, or bass, or contralto, but much practice on the extremes of the voice is unadvisable.

If pure tones are produced in the medium range of the voice the highest or lowest tones will be found ready when called for.

Therefore practise the extremes of the voice only enough to know the limits of the voice and to be assured the tones are there.
When the singer can perform the preceding simple exercises and know that the tones are all focused, or placed and delivered, precisely alike, he is ready to practise any scale, down or up, and to execute any musical exercise or song for which he is intellectually fitted..