A NEW APPROACH TO THE CLASSIFICATION OF ODOR* By JAMES J. w. MIDDLETON yacqueline Cochran, Inc., Newark 5, N. No present classification sys- tem has the elements of academic or pragmatic utility or validity, since all are based on incorrect premises. The methods used in the past in devising these systems take no cognizance of the physiological mechanism of odor detection in any way, but merely set up arbitrary "components" of odors, which possess a very high probability of being invalid. Surely there must be a more rigorous or methodical way of arriving at our underlying conceptions of the true basis of odor sensation. Assuming that odors are made up of a mixture of "pure odor sensa- tions" of whatever number and whatever type, the gamut of odor is not indefinite or infinite. Individ- ual sounds or tones, for instance, are limited to the range between the low and high limits imposed by the ear. Combinations and inten- sities, however, may actually be infinite in number and complexity. * Based on a discussion of odor by the author at the "Symposium on Odor Classi- fication" presented at the December 8, 1949, Meeting, New York City. The same is true in color, our range being limited by our visual detect- ing mechanism, although the inter- play of combinations of frequencies varying in number and relative in- tensities may be infinite. It would seem desirable, for the purpose of tracking down the method of odor detection physio- logically, to eliminate, until some future time, any considerations of emotional reactions to odors as being pleasant or unpleasant. For instance, it does seem possible that a few of the inaccuracies experienced in classifying an odor by the Crocker-Henderson system might be due to one of the odors being more unpleasant than another, and therefore being given a higher and "incorrect" figure for the unpleasant component of "caprylic," for ex- ample. We can, in essence, detect individ- ual colors or tones, complex though each may be, without any appreci- able emotional response. All are more or less equally unemotional by themselves, taken one at a time, instantaneously. We get into trouble when we examine relation-

22 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ships between two or more colors or tones, with emotional involvements. Here, the two fields are rife with theories of harmonies, which the- ories are at variance with each other. As a matter of fact, in order for present theories of musical harmony to exist with a greater de- gree of self-consistency, it first be- came necessary to change, for psy- chological purposes, our very basis of sound, the scale. True harmonies in the scientific frequency scale do not affect the ear as harmonies in even a majority of cases. It was necessary to change the frequencies of some of the scale notes into the even-tempered scale, after which simple ratios determining harmony were possible to discern. Possibly the ear itself is "equal tempered." I know of no analogous situation in color work, unless it be that until we first defined our color scale, or color wheel, by using the eye itself as the basis, we got nowhere. The best wheel is that dependent upon the phenomenon of after-vision in the eye, viz., long staring at one color until perceptual fatigue has set in, then noting the complemen- tary color which seems to appear on a sheet of white paper to which the gaze is shifted. There is one basic question which must first be decided before one can proceed further with an analysis of the odor sense. Fortunately, we have in two of our other senses an analogy for each of the two pres- ently conceivable modes of sense reaction. One mode is exemplified by the ear. It is a well-demonstrated physio- logical fact that a nerve, which must intervene in any sense im- pression, can do only one thing-- conduct a "message" of undefined nature. A single nerve cannot an- alyze or transmit anything but one simple "message" at a time. The only variations possible are those of intensity, which also includes a zero intensity, or no sensation at all. Groups of nerves have one possible additional function, that of transmitting a finite number of different messages, according to the precise mechanism of detection employed at the sensing end of any nerve fiber. The ear solves this dual possibility in an opposite way from the eye. The ear, according to present theory, analyzes an in- stantaneous set of tones into its component frequencies by virtue of its mechanical construction as a res- onating chamber which, by its very dimensional variation, causes any individual volume or area of the chamber to resonate at one fre- quency, and nerves situated there to accept perforce that particular fre- quency as their stimulation. Leav- ing out, for the time being, the all- or-none principle, the group of nerves at that spot can also trans- mit perception of the intensity of that precise tonal frequency. Thus, here we have a mechanism which makes a complete ultimate qualita- tive and quantitative analysis of the instantaneous impression. The eye functions differently. The eye does not perform a com- plete analysis in the same connora-