The bolometric examination of sun spots shows that they, too, exhibit much greater absorption at the violent end of the spectrum than in the infra-red. Thus, if our eye were like the bolometer, and could view sun spots by homogeneous rays of different wave lengths of the infra-red as well as violet, we should see the same spot four times as dark in violet light as when viewed by extreme infra-red rays.

In all the observations and reductions involved in the work described above Mr. Fowle has taken by far the greatest share.

The

The temperature data plotted in Plate V are reduced from the Internationaler Dekadenberichte, published by the Kaiserliche Marine Deutsche Seewarte. reductions were made partly by R. Norris and partly by J. Dwyer.

(2) MISCELLANEOUS WORK.

Radiation of the stars.

Preliminary preparations were made for the detection of the radiation of the brighter stars. It was at first thought practicable to mount the bolometer in the center of the tube of the 50-centimeter diameter mirror of 1-meter focus and to point the mirror directly upon the star to be examined, but it was quickly found that the disturbances due to exposure to outside air were too great to be permissible with the refined sensitiveness of the bolometric apparatus. Afterwards the mirror and bolometer were placed within the inner chamber of the observatory, and the starlight was reflected in from a 30-inch plane mirror on the coelostat. The galvanometer employed was the one described at pages 91-92 of your report for the year ending June 30, 1902.

The sensitiveness available depends largely on reducing the damping of the needle, and a long time was spent in making the galvanometer case air-tight, so that a pressure of 1/1000 atmosphere or less could be maintained without rapid change. In this we were at length so successful that the change of pressure was hardly appreciable in several months. Great difficulty was encountered in balancing the bolometric apparatus at the high sensitiveness employed on account of small electromotive forces in the galvanometer circuit and its shunts. Thus a balance would be obtained with a certain shunt across the galvanometer, and on passing to the next shunt a very great deflection would be found, due to a new electromotive force in the new shunt circuit. At length it proved necessary to discard shunts wholly, and to employ instead a variable resistance in series with the galvanometer. Great difficulty was still experienced in balancing, but not so great as to render it impossible, as before. When once balanced the apparatus was well behaved. The sensitiveness appeared from tests on candle flames to be very much greater than that obtained by previous experimenters on stellar radiation. Unfortunately the difficulties encountered were not surmounted until early in January, and bad weather prevented a trial on the stars until more important work displaced the investigation for the present.

New apparatus.

Bolometer.-In our previous construction of bolometers we have been guided more by experience than by any theory in their design. While preparing for the research on stellar variation it seemed very desirable to determine the conditions which would insure the highest sensitiveness. Accordingly the subject was studied from the standpoint of Fourier's analytical theory of heat, and numerous experiments were made to further enlighten it. As a result a complete theory of bolometer construction was reached, and it is now possible

to know in advance how to design and construct a bolometer to give the best result under stated conditions.

In accordance with the results thus reached, a vacuum bolometer was constructed for the observation of stellar heat which is several times as sensitive for the purpose as the best bolometer hitherto prepared here.

Cœlostat.-- A new cœlostat with two 15-inch mirrors was obtained for the proposed expedition to a high-altitude station. The general design was prepared at the Observatory, and the construction was by Mr. M. E. Kahler, of Washington, excepting some work done on the clock at the Observatory shop. A photograph of the apparatus as in use at Mount Wilson is here shown (Plate VII).

Several unusual features may be noted. The rotating mirror is shown mounted on a carriage which may travel on ways either east and west or north and south, so as to be adjusted for the position of the sun at any time in the year. The carriage is of the same height as the lower base of the support of the second mirror, and the two mirrors may be interchanged if desired, so that the rotating one can be fixed in one place and the eastand-west and north-and-south motions can be made with the other. The driving clock is fastened to the support of the polar axis and designed to run equally well in any position, so that the adjustment of the axis for different latitudes can be made without altering the driving mechanism. In order to conform to the motion of the carriage, the clock is driven by springs instead of weights. At your suggestion there was introduced a driving spring, which is itself kept wound to a nearly constant tension by two larger springs. Thus the driving force is uniform though the two larger springs run down. The rate of their unwinding is governed by an escapement driven from the governor train. The governor itself is of the centrifugal type, but has springs instead of gravity as the governing force, and will thus run in any position. The accuracy of the clock is ample, so that the reflected beam from the cœlostat frequently remains constant in direction within one minute of are for a half hour.

Standard pyrheliometer.--Mention was made in my last year's report of a new form of pyrheliometer then under construction. This instrument has been completed and forms a part of the equipment of the Mount Wilson expedition. It appears to justify all the hopes that had been connected with it. As stated last year, it receives solar rays in a hollow blackened chamber of the shape of a test tube, from which little radiation can escape by reflection, or heat by convection, owing to the deepness of the chamber and to its numerous blackened diaphragms. A current of water circulates around this chamber and takes up the heat absorbed on its walls. Platinum resistance wires serve to determine the temperature of the water before and after its passage around the chamber. Thus the sun's rays entering a known aperture produce a measurable rise of temperature in a known amount of water. A certain check is had on the accuracy of the measurement, for a coil of wire is introduced within the rear end of the chamber, and in this coil heat may be produced electrically at a known rate. This heat warms the air and indirectly reaches the walls of the chamber, and may be measured as if it were produced by radiation.

At nine recent trials made on several different days, with the instrument in different positions and with different rates of flow of the water current, the heat" found" ran between 97 per cent and 103 per cent of the heat introduced. The mean of nine trials gave 100.4 per cent "found." If, then, the electrically supplied heat is thus closely measurable, much more should that