I was reading the second chapter of Three Areas of Experimental Phonetics by Peter Ladefoged (1967), in which he summarizes the studies he conducted in the 1950s and 1960s which demonstrated practical limitations of Daniel Jones's Cardinal Vowel system.
In it, I was struck by the fact that Ladefoged couldn't ascertain the frequency values of the first and second formants of high and back vowels because for high vowels "the first formant has a very low frequency" and for back vowels "the first and second formants are too close together" (p. 101). He then goes on to say this difficulty may have bearing on our perception of vowel quality, one effect of which being that the distance between the cardinal [i] and [e] is smaller than those of some other combinations (p. 103).
The chapter concludes, albeit tentatively, that (pp. 132-3):
(1) The acoustic quality of most vowel sounds can be conveniently specified by stating the frequencies of their first two or three formants.
(2) This is not true of vowels which are called in traditional terms close vowels, nor of so-called back vowels. It is not at all easy to analyse these vowels in terms of their formants.
(3) The perceptual quality of a vowel usually depends on the relationship between the pitches of the formants of that vowel and the pitches of the formants of other vowels pronounced by the same speaker.
(4) The listener to speech uses his past experience to form an adaptation level, the immediate past experience of a particular voice being the most important factor in this process.
(5) Neither of points 3 and 4 above has been shown to be true for the vowels mentioned in 2 above.
But as far as I know, linguists of today have no problem getting the F1 and F2 of high and back vowels. Is point 2 above still true? If not, why was it so difficult? And what made it easy (e.g. some kind of technological advancement)?
And what about point 5? It is still true that the acoustic distances between the cardinal [i] and [e] and between [u] and [o] are greater than those between [e] and [ɛ] and between [o] and [ɔ]. Is this just a random coincidence on the part of Daniel Jones or perhaps the phonology of French or German, or are these values indeed perceptually (more or less) equidistant? AFAIK the latter is true, but if so, was Ladefoged's assumption (in 1967) that the incongruity between acoustics and perception stems from certain formant frequencies being too close to one another correct? (EDIT: I'll ask this point separately here.)