Head Resonance
We have found many students who were unable to combine head resonance with their ordinary tone, though each could be made separately.
In such cases, if the following directions are accurately followed, the desired result will be obtained in a few days. While singing a full round tone on the vowel "o," pinch the nostrils until they are fully closed. Now change the vowel sound to "oo," as in pool but do not alter the position of the lips; the result is a nasal sound full of head resonance. Now release the nostrils, and the vowel sound will return to "o;" moreover, the head resonance will remain in the tone with no trace of nasality.
Except the "ng," there is no other nasal consonant sound. In producing this the position of the tongue against the soft palate prevents free head resonance. The tongue also fills the larger part of the mouth, and thus prevents all natural development of the tone; hence this sound is to be avoided.
Very few singers attain perfection in tone production, but in one's study the head resonance must not be neglected, if a symmetrical development is expected.
—George E. Thorp, W. M. Nicholl, Text Book on the Natural Use of the Voice(Robert Cocks & Co., London) 40-41.
William Nicholl and his curious book have appeared on these pages before (click on his label to find my previous post), the even more curious thing being Nicholl was a student of Manuel García.
Those who read the full text will find that Nicholl, like many others during his time, believed that the soft palate should not totally seal off the passage from the mouth to the nasal passage—which is, of course, an explanation—no matter how wrong—of voice placement. Physiological explanations aside, the idea that one should sing with head resonance is a very old idea indeed, and one that is a hallmark of the old Italian school.
Of course, when I read this passage, I could not help thinking of my own teacher who would often demonstrate to students in substance what Nicholl suggests above, which is the articulation of vowels with a non-moving mouth. Extraordinary to witness, she would say: "This is how you make your vowels!" and then intone [i] [e] [a] [o] [u] with a barely opened mouth and hardly any movement of the lips and jaw—thus illustrating García's assertion that the true opening of the mouth was the pharynx.