I have only just time to send what you want over to Liverpool this afternoon, and to acknowledge your letter of April 14th with enclosures, which are all very welcome. … The diffused and direct sun experiments are very interesting. They differ _in toto_ from the Heidelberg results. Pray get some more at low elevations of the sun. I must now close … as the _Jerome_ sails early in the morning. P.S.—I hope you may be able to get _one_ cloudless day before you leave, as the clouds evidently much modify the result. It is almost a pity that you did not go out in the vacation for September and October, but it cannot be helped now. If you get a cloudless day begin early, take four or six sets of observations (one at noon, of course) until late in the evening, so as to get the _low_ elevations. The Kew observations showed that the mean chemical intensity for hours equidistant from noon is practically the same on the same day, and that the daily maximum of chemical intensity corresponds with the maximum of solar altitude. Measurements showing the daily rise and fall of chemical intensity for each of the twenty-four months were obtained, as well as of the biennial variation for the same period. It was pointed out that the curve of yearly chemical intensity is not symmetrical about the vernal and autumnal equinoxes. Thus for 100 chemically active rays falling at the spring equinox at Kew, there fell at the autumnal equinox 167 rays, the sun’s mean altitude being the same, the difference being probably due to the greater atmospheric opalescence in the spring. The Pará observations were interesting from the fact that they were the first measurements of photometric intensity made within the tropics, and that they served to dispel certain fallacies about photographic effects in very hot climates at that time current. The observations showed that the relation between the sun’s altitude and chemical intensity may be represented by the equation: C I _a_ = C I₀ + const. _a_, where C I _a_ represents the chemical intensity at a given altitude _a_ in circular measure, C I₀ the chemical intensity at the altitude 0, and const. _a_ a number to be calculated from the measurements. Comparisons between the observations at Kew and at Pará on the same days in April showed that the daily mean chemical intensity at the latter place was from ten to fifty times greater than at Kew, the wide differences being due to the enormous and rapid variations in intensity from hour to hour which the chemically active rays experience in the tropics during the rainy season of the year.[9]