A novel theory of primary olfactory reception is described. It proposes that 
olfactory receptors respond not to the shape of the molecules but to their 
vibrations. It differs from previous vibrational theories (Dyson, Wright) in 
providing a detailed and plausible mechanism for biological transduction of 
molecular vibrations: inelastic electron tunnelling. Elements of the tunnelling 
spectroscope are identified in putative olfactory receptors and their associated 
G-protein. Means of calculating electron tunnelling spectra of odorant molecules 
are described. Several examples are given of correlations between tunnelling 
spectrum and odour in structurally unrelated molecules. As predicted, molecules 
of very similar shape but differing in vibrations smell different. The most 
striking instance is that of pure acetophenone and its fully deuterated analogue 
acetophenone-d8, which smell different despite being identical in structure. 
This fact cannot, it seems, be explained by structure-based theories of odour. 
The evidence presented here suggests instead that olfaction, like colour vision 
and hearing, is a spectral sense.