A, culture cell (quartz floor, &c.) arranged as usual; B, an inverted matchbox painted black and with a hole over the hanging drop; C, a quartz bottle screen of water or bichromate, placed over the hole, so that no light can reach the hanging drop except through the screen.

and support the (quartz) screen. Each stood side by side, with a dummy cell provided with a thermometer, one pair (culture and dummy) under bichromate, the other under water.

They were on a table outside the north laboratory, exposed to the blue sky, but with no trace of direct sunlight. The amount of white cloud increased from 10.45 to 11.15, and very little open sky was available, except at intervals. The details concerning temperaturo and weather are given in the following table:

:

At 7 P.M. all were brought into the laboratory, and the temperature rose to 16 25° at 7.5, 18° at 7.20, 18° at 8.45.

The cultures were examined at 4.40, 7 P.M., and 8.45, in each case with no traces of germinated rodlets in that under water, whereas that under bichromate had germinated out normally and grown well,

as the table shows.

Bichromate.

Water.

Time.

Average. Extremes. Mean. Temp. Average. Extremes. Mean. Temp.

How the conclusion that the light from a sky only occasionally blue, and generally very cloudy, retarded the spores indefinitely is to be avoided here seems inconceiveable. There is nothing in the minute differences of temperature--the maximum at 11.45 was only 04 of a degree-to account for it, and as for the variations (from 17.5 to 23-25), even if they had attained dangerous ranges, they were similar in both cases; but they were in no way dangerous; the range was really very favourable.

On August 4 this experiment was repeated, but with broth instead of 1 per cent. gelatine drops. The cultures were started at 10.15, and kept till 10.55, at a north window in the laboratory, because rain was threatened. The sky then became blue in patches -a very deep blue, but with rolling heavy cumulus clouds-and the cultures were then taken outside and exposed exactly as yester day, all the cells standing on a glass mirror, the surface of which was covered with black paper except just below the drops and the thermometer bulbs. The variations of sky and temperature are given in the annexed table.

The results of examinations at 2.57 P.M., 4 P.M., &c., are given in the annexed table. They prove conclusively the evident light

action.

Time.

Bichromate.

Water.

Average. Extremes. Mean. Average. Extremes.

Mean.

If we take the doubling periods of the bichromate-culture, we have

(averages)

2.57 P.M.-4.8 P.M., i.e., 71 minutes at about 19 75-17 75° C.

= = 4.8
= 5.15
6.45

Of course, we must remember these numbers are for averages of lengths, and cannot be compared too closely with the numbers got as averages of detailed observations. Nevertheless, there is a certain and more than merely rough agreement, which suggests that the growth was normal for the rather wide ranges of temperature.

May 22 opened very clear and blue, but with a cold N.E. wind; after 10 A.M. it clouded over, and the afternoon was dull and wet as well as cold.

A culture of spores kept at 12° C. overnight was germinating freely, but slowly. The cell had a quartz floor; the drop was brothgelatine.

At 10 A.M. this was put over a concave silver mirror and quartz cell of water, and the measurements begun. The exposure was to the open blue sky to the south; no direct sunshine at all was allowed, a cardboard screen being used.

The measurements were as follows, the temperatures being those of a similar cell over a plane silver and bichromate quartz cell; experiments showed this registered the same temperatures.

The experiment was simply a trial to satisfy myself that the apparatus worked satisfactorily. As the following table shows, the growth was very slow, but six hours' exposure to the dull light did not kill the filaments.

This filament doubled its length in 5 hours and 28 minutes at 12-13-12° C.

On June 21, which turned out a brilliant sunny day, with blue sky, a little hazy, and very hot, the following experiments were made; the apparatus being quartz cells, and screens, and double cardboard shades to microscopes arranged as in the accompanying diagram (p. 110).

The spores, sown in broth-gelatine the night before at 12° C., had germinated evenly to rodlets by 9 A.M., and were exposed at the open south window at 9.45, over screens of water and of K. bichromate, the silver mirrors being turned to the blue sky to the south.

The window was closed from 2.15 P.M. onwards.