Chen 臻 and Chen 真, Chih 櫛 and Chih 質 when added together, each pair forms a complete third division rime, but the Ch'ieh-yun 切韻 separates each pair into two rimes.  The rime tables place Chen 臻 and Chih 櫛 in the second division, so the separation of Chen 臻 and Chen 真, Chih 櫛 and Chih 質 would seem to be the difference between second division rimes and third division rimes.  However, Chen 臻 and Chih 櫛 only have ch'ih-yin (齒音: supradental sibilants).  This is unparalleled by any other second division rime. In my article, "On the Readings of the Two Groups of Upper Fan-ch'ieh Characters (反切上字) of the Ancient Chinese Initials Chao, Ch'uan, Ch'uang and Shen (照穿床審)," I determined that Chen 臻 and Chih 櫛 represent the characters of the Chuang 莊 series of initials of the Chen 真 and Chih 質 rimes.  Because the Chuang 莊 and Chang 章 series of initials in third division rimes have different medials, i.e., -e- and -j-, the Ch'ieh-yun has a basis for separating Chen 臻 and Chen 真, Chih 櫛 and Chih 質.  Thus the contradiction stated above is eliminated. 

In this paper I wish to further point out that in the entire Ching-tien shih-wen 經典釋文, all characters belonging to the two rimes Chen 臻 and Chih 櫛 use characters belonging to the Chen 真 and Chih 質 rimes respectively as their lower fan-ch'ieh characters (反切下字).  Moreover, independent second division rimes do not use characters belonging to third division rimes of the same rime chart 轉 as their lower fan-ch'ieh characters, but second division Keng 庚 rime characters use third division Keng 庚 rime characters for lower fan-ch'ieh characters; in both of these respects the Ching-tien shih-wen agrees with the Ch'ieh-yun.  Thus we can see that Chen 臻 and Chih 櫛 are really a portion of Chen 真 and Chih 質 respectively, and the only difference that we can ascribe to their finals is that of medial.  We cannot say that Chen 臻 and Chih 櫛 lack a medial, because otherwise the Chuang 莊 series of characters of the Yang 陽 and You 尤 rimes ought to appear under the T'ang 唐 and Hou 侯 rimes. Thus the difference between Chen 臻 and Chen 真, Chih 櫛 and Chih 質 can only be ascribed to a difference in medial; the medial of Chen 臻 and Chih 櫛 is here reconstructed as -e-.

As the factor of whether Chen 臻 and Chih 櫛 are independent rimes or not does not affect a determination of their phonetic value, we can conclude that all characters placed in the second division of the rime tables have a medial -e-.  From this it is not hard to deduce that all characters placed in the fourth division of the rime tables have the medial -i-.