480 SECTIONAL TRANSACTIONS.—I. 
to cause relaxation (as experiments carried out in conjunction with Dr. 
S. W. F. Underhill have shown), change of reaction of the muscle appears not 
to provide an explanation of the phenomenon of contraction. A more feasible 
explanation seems to be that some potent metabolite is responsible for the 
contraction, that the circulation in the contracted tissue is greatly reduced, with 
the result that lactic acid and carbon dioxide accumulate, so causing relaxation. 
15. Prof. H. BE. Roar.—Measurement of Colour Blindness in terms 
of Wave-lengths. 
16. Dr. F. W. Evrmce-Green, C.B.E.—The Effect of the Blood in 
the Retina on Colour Equations. 
Whilst the effect of the pigment of the yellow spot has been frequently 
discussed, that due to the absorption by the blood in the retina has been 
generally overlooked. It is known that the phenomena attributed to the visual 
purple of the rods gradually diminish in the rod-free portion of the retina. 
Curves constructed by Hecht show that the rod-free portion behaves as if 
there were dilute visual purple in this region; this would naturally follow if 
the visual purple has to flow into the liquid surrounding the cones of the fovea. 
Numerous equations are valid both for the fovea and the peripheral portion of 
the retina, but if an equation be made of red A 650, (corresponding to an 
absorption band of hemoglobin) and green A 553.8un to match the white light 
of the tungsten arc, it will be found that more green is required in the equation 
when the image falls on the retina 15° from the centre of the fovea, in which 
there are no blood-vessels. | 
17. Dr. M. C. Grasuam.—Dental Caries at Porto Sanio. 
The object of this paper is to stimulate inquiry and to suggest that the 
mineral waters of the island possess some influence in resisting the development 
of caries. Porto Santo is a small island of the Madeira Archipelago, and its 
water-springs are highly mineralised with chlorides, carbonates, and sulphates, 
in. contrast to the sweet waters of the principal island. The outstanding 
features of the people’s diet are that the food is taken cold, no green vegetables 
or milk are included, and there is nothing to require grinding mastication. 
The people drink moderately. Consumption is frequently present, while there 
is no scurvy, no alimentary disorders, and no malignant disease as far as the | 
author can ascertain. But the teeth of the district are characterised by a thin 
yellow line across the upper incisors, which in after-life spreads and stains 
the teeth generally. The stain is unknown in Madeira. The writer states with” 
confidence that the yellow stain on the incisors is a sure indication of a sound” 
set of tecth, and its regular occurrence is believed to furnish conclusive 
evidence of the permeation of the blood fluids in the interstices of the columnar 
enamel, and is certainly due to some constituent in the local and highly 
mineralised water. A 
18. Prof. J. J. BR. Macueop, F.R.S.—Lecture on Insulin and 
its Value in Medicine, followed by discussion. 
19. Mr. J. C. WatitEer.—Conditions which Determine the Direction 
of the Photo-Hlectric Current in Green Leaves. f 
A. D. Waller showed that the electric effect of illumination upon the green 
leaves of Iris, Begonia, and Nicotiana is negativity of the illuminated part ; in 
the green leaves of Mathiola and Tropeolum the effect is positivity of the 
illuminated part. - 
Photographic records of the two types have been obtained, using Geranium 
(which belongs to the Iris type) and Tropzeolum. %* 
The type of response in Geranium can be brought to resemble that of 
Tropeolum by keeping the plant for long periods in darkness. yi 
The type of response of Tropxolum resembles that of Geranium under certain 
conditions not yet defined, but apparently connected with the previous action 
of sun and wind. Ht 
