Viewing the Hering theory from a physical standpoint, and in the light of colour measurement, it appears to be deficient in several respects. To take one point. We have seen that when blue and yellow are mixed together to make white the sum of the luminosities of the two colours separately is equal to the luminosity of the white produced. According to the Hering theory, the yellow colour contains a certain amount of the white-black sensation besides the yellow sensation, as does also the blue colour besides the blue sensation. The theory tells us that when white is produced by the mixture, the blue sensation undoes the work that the yellow sensation has done, and the white sensation is alone left behind. If this be the case, the sum of the separate luminosities cannot be the same as that of the white produced, but should be greater. The theory also has to be strained sometimes to make it fit in with other observed facts. Take, for instance, the case of persons who are called red-blind and green-blind on the Young theory. We are told by the Hering theory that both are red-green-blind--that is, blind to both green and red, and only see blue and yellow--and that the only difference between them is that the former has his spectrum slightly shortened at the red end, the maxima of the yellow-blue sensations being shifted a little further towards the violet end of the spectrum. The natural question to ask is: Why this shift occurs? Surely it is more rational to adopt a theory which does not require such a supposition? If the sensitive matter acted upon by the yellow-blue rays be always of the same chemical composition, the shift cannot occur. It might, perhaps, be allowed that one shift was practicable, but, unfortunately, the shifts must become numerous when the cases of partial colour blindness are to be accounted for, and this would necessitate a constantly varying chemical composition of this matter, and of that acted upon by the red-green rays. Again, in the extinction of the spectrum, the red and the green sensations in quantities to neutralize one another should be extinguished nearly together, even allowing for what physiologists tell us is the case, that the breaking down, or dissimulation, of cell tissue continues longer than its building up, but we find a large difference between the two. As already indicated, the luminosity curve of the feeble spectrum favours the theory of Hering being that here we only have the white-black sensation, and naturally the persistency curves must be scored in its favour. But the cases of B. C. and M., it seems to me, cannot be explained by the theory without any undue straining or assumptions. If we try and fit the cases of colour blindness due to tobacco scotoma to the theory, we find that in many cases yellow is not recognised, though blue is invariably. If the blue be active, the yellow should also be so.