canvas hut light-tight. The canvas was put on outside the frame- work, and the roof was thatched with " alang-alang " on the top of the canvas, to help to keep out rain and sun, and the thick paper was tacked in position inside the framework. This was done in such cunning fashion under Dr. Wallace's direction, that the hut was well ventilated, and I doubt if so convenient and comfortable a dark room has ever been constructed in the tropics.
With reference to the dark room, I may add that a good supply of ice for keeping the chemical and washing baths cool was obtained from Padang, but this was only used for the plates that had to be preserved. A great many of those taken in the process of adjusting the instru- ments were entirely rotted when ice was not used, but as these were only of passing value there was no object in preserving them.
9. General Arrangement of the Instruments.
Inside the large hut three piers were built :
(i.) One to carry the 4-prism spectroscope and the visual objective grating ; top level and oriented north and south.
(ii.) Another immediately to the west of it to carry the ccelostat ; top level and oriented north and south.
(iii.) A third, still more to the west, to carry the double tube camera, the photographic objective-grating camera and other apparatus, all inclined and pointing downwards towards the cxelostat ; top inclined at an angle 22 with the horizon, and in azimuth 70 west of south.
The hut was set so that one diagonal of the base was parallel to the length of the third pier, or in other words, to the double tube camera ; the long sides of the hut thus lay nearly east and west. The part of the roof over the N.E. corner was covered with the movable corrugated iron, so as to admit the sunlight to the coalostat and 4-prism spectroscope below.
The two clocks for driving the ccelostat and the 4-prism spectroscope were put on piers just outside the north side of the hut, and were protected by sheets of corrugated iron leaning against that wall of the hut. The weights for the ccelostat clock were hung over a pulley attached to the bamboo framework that supported the thatch.
The visual polariscopes were fitted to the framework of the hut up in the N.E. corner.
10. The Ccelostat.
The 16-inch ccelostat, which was used by Professor Turner in India in 1898, and in Algiers in 1900, had been specially provided, by the care of Dr. Common, with an additional support to the mounting, which made it quite stable and suitable for use on the equator. The
