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THE FORTY-FOOT CAMERA AND INTRAMERCURIAL APPARATUS AT ELEPHANTINE ISLAND.

disturbing surface conditions, the range of temperature day after day amounting to less than a degree Fahrenheit.

The last ten days of our stay at Aswan proved the most trying that we had, for the winds dropped and the humidity rose above fifty. Operations in the dark-room were necessarily restricted to night and early morning, and the behavior of the chemicals in the heat and the uninterpreted action of other agencies added perplexities to the work of development. Our dark-rooms were improvised without sinks or running The hotel gave us the unlimited use of its filters, and the Survey Department sent up generous supplies of distilled water from Cairo, while the willing labor of our ABBAS and MOHAMMED furnished the Oriental equivalent of Yankee conveniences. Ice was an essential, of course, brought by train from Cairo daily.

water.

Nineteen photographs were obtained, having exposures varying from half a second for the inner corona, to sixty-four seconds for the fainter outlying streamers. With the intramercurial apparatus the time of totality was divided as nearly as might be to obtain duplicate plates along the ecliptic in the vicinity of the Sun. The exposure with the spectrograph lasted throughout totality, except for a second or two at the beginning and end.

No detailed study of photographs is made at an eclipse camp. This requires the resources of measuring-engines, microscopes, comparison-plates, and other records. At the observing station the one object is to bring out all the detail the plates will yield and fix them against the chance of accident from light or chemical change. They are then packed in their original boxes, separated at their edges by strips of paper. These are then sealed in tin, and put in a strong wooden box, excelsior lined. This is then packed in one stronger still, and labeled, in this case in three languages, English, French, and Arabic, for shipment to Mt. Hamilton.

Eclipse successes and failures have always a stimulating effect. By the following of clues in the earnest endeavor to learn more of the many things that formerly could be studied only during the brief moments of totality the effective means of research have been greatly extended and have given us what may be called indirect eclipse results quantitatively more

numerous perhaps than those which have been obtained directly. Holding in mind the great practical importance of a more complete understanding of the Sun and the high probability of light being thrown upon its problems by observations that can at present be made only during the brief and long-separated total eclipses, it is apparent that every opportunity should be embraced to make the most of these occasions. It is to the credit of the Lick Observatory that it has played a conspicuous part in the observation of eclipses of the Sun, and not too much can be said in commendation of the systematic giving by which Mr. WILLIAM H. CROCKER, and, before him, the late Colonel CHARLES F. CROCKER, have made possible the continuous study and investigation of these phenomena by that institution.

DETROIT OBSERVATORY, UNIVERSITY OF MICHIGAN,
ANN ARBOR, January 22, 1906.

NOTE ON ANOMALOUS REFRACTION.

BY FRANK SCHLESINGER AND G. B. BLAIR.

Under normal conditions atmospheric strata of uniform density lie parallel to the Earth's surface. In this case the expression for refraction takes the well-known form k tan 2, in which is the true zenith-distance of the object and k is a quantity that varies slowly with the zenith-distance, the temperature, and the height of the barometer. For present purposes, however, we may regard k as a constant and equal to 57".

Let us now consider the effect upon the refraction of a small inclination in the strata of uniform density. Imagine a normal to be drawn to these strata, and let and a be respectively the zenith-distance and azimuth of the point at which this normal pierces the celestial sphere. This point is evidently the origin from which zenith-distances should be reckoned for the computation of the refraction under the assumed conditions. Consequently for in the above formula

we should substitute (with a sufficient degree of approximation) - cos (a-A), A being the azimuth of the object. Hence the correction arising from anomalous refraction is

z

k tan [cos (a — A)] — k tan z

or very nearly k. cos (a—A) sec2 &

For objects in the meridian this becomes

kcos a sec2 z.

(1)

(2)

The positive or negative sign is to be used according as the object is south or north of the zenith.

The following table gives the values for various zenithdistances of the coefficient of cos a, the latter being expressed in minutes of arc:

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This table shows that the effect of anomalous refraction is nearly the same for all objects that culminate within 30° of the zenith, but that it increases rapidly when the zenithdistance surpasses 50°.

It will be instructive to apply the above formula to actual observations. So far as we know, the only ones suitable for the purpose are those which have been made at the six international latitude stations. The programme for observing at these stations includes, in each night's work, twelve pairs at small zenith-distances (never more than 23°), and four pairs at large zenith-distances1 (about 60°). An ordinary programme-that is, one which contains pairs at small zenithdistances only-cannot be used for our purpose, since, as we have seen from Table I, the effect would be nearly the

1 For brevity we shall refer to these as refraction pairs and to those near the zenith as zenith pairs.

same for all the pairs, and it would be impracticable to distinguish a refraction effect from any other disturbing phenomenon. Furthermore, it would be equally futile to attempt to deduce the value of cos a from absolute measures of declinations, since these do not possess the requisite accuracy.

The results of the first two years of the international work have been published by Dr. ALBRECHT in volume I of the "Resultate des Internationalen Breitendienstes." On the data given in this memoir Table II is based.

Column I gives the name of the station and column 2 the number (n) of nights during 1900 and 1901 on which all four of the refraction pairs, and at least ten of the twelve zenith pairs, were observed. No use is made of the other nights in the present paper, which, even with these restrictions, covers more than fourteen thousand separate determinations of the latitude. Column 3 contains the sums of the squares of the "zenith divergences" obtained thus: ALBRECHT'S Plate XI shows with red lines the definitive latitudes at the six stations; and on pages 130 to 139 are given the results for each night from the zenith pairs. The difference between these two quantities was taken for each night, and the sum of their squares for the whole two years is entered in column 3.

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Let be the mean error of the definitive latitude shown in ALBRECHT'S Plate XI.

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ρ,

the mean error of the mean of the ten to twelve
zenith pairs observed each night.

the mean effect of anomalous refraction at the zenith.

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