106 



KNOWLEDGE. 



[May 1, 1900. 



the bands may be correctly judged. A rod should be 

 placed to mark the direction of the bands themselves, as 

 seen at the beginning of totality ; and another to mark 

 their direction of motion ; another pair being used for 

 a similar purpose for the bands seen at the close of 

 totality; and after totality is over the most careful 

 determinations must be made of the directions of the 

 rods, and of the position of the sheet or wall. The 

 following questions drawn up by Mr. E. W. Johnson 

 for the assistance of the Members of the British Astro- 

 nomical Association should be answered. 



Question.'*. 



1. How long before totality did the bands appear? 



2. What number of bauds were visible, say, in 10 

 seconds ? 



3. What was the direction of motion? 



4. Where they inclined to the direction of motion ? 



5. What was the direction and force of the wind? 



6. Did they come unifonnly, or in batches? 



7. What was their speed ? 



8. What was the width of the bands? 



9. What was the distance apart of the bands ? 



10. Were they very faint, or clearly defined? 



11. Was their direction after totality the same as 

 before ? 



12. How long after were they visible? 



13. Did you see any bands during totality? 



The subject of shadow-bands leads naturally to 

 meteorological work, for there is no doubt that the 

 direction of the wind affects the direction of the motion 

 of the bands. The meteorological observer should there- 

 fore provide himself with some form of vane and some 

 means of ascertaining the force and speed of the wird. 

 The wet and dry bulb thermometers would seem to be 

 the next most important instruments to take, that the 

 change in temperature and in saturation of the air 

 might be marked. The barometer would come in the 

 third place. It is of course desirable that observations 

 should be made at regular intervals for some days both 

 preceding and following the eclipse, especially at the 

 same hour of the day as that when the eclipse takes 

 place. 



Those who find themselves about to witness the eclipse 

 yet without any instruments or any preparations for 

 observing should not let it pass wit'aout some record. They 

 should note the appearance of the sweep of the shadow 

 over the country as it comes and as it goes; the colours 

 of sky, land and sea should also be noted, the sky being 

 divided into three regions — namely, overhead, at sun- 

 light, and near horizon. 



Photogiaphic cameras come very close to a definition 

 of a telescope, and hence should be excluded from the 

 scope of the present paper. Yet as in all probability 

 there are some hundreds of possessors of cameras for every 

 one who possesses an astronomical telescope, it is perhaps 

 not superfluous to remind photographers that a very large 

 field is open to them. Cameras with a focal length "of two 

 feet and upwards may be profitably used upon the corona 

 itself. In this case the camera should be firmly fixed 

 and exposures not exceeding half a second should be 

 given. If the focal length be not more than 15 times 

 the aperture, this with an extra rapid plate will probably 

 be found quite sufficient. For shorter focal lengths 

 shorter exposures or slower plates may be used. 



Hand cameras may be profitably employed for photo- 

 graphing the landscape dui-ing the approach and re- 

 cession of the shadow. A series of photographs taken 



at five minute. intervals with a uniform speed of shutter, 

 such as Miss Bacon took at Buxar in India, would give 

 a very interesting and certainly very pretty record of 

 the increase in the darkness as the eclipse comes on. 



Finally, a valuable record of the total light of the 

 eclipse can be obtained by exposing a plate in a pi'inting 

 frame to the light of the corona during totality. Por- 

 tions of the plate can be exposed for different lengths 

 of time or the plate itself may be placed under some 

 form of sensitometer. Further information would be 

 obtained by using as well different coloured screens in 

 connection with plates of various coloured sensitiveness. 



A PHOTOGRAPHIC SEARCH FOR AN 

 INTERMERCURIAL PLANET. 



It has not been, in general, the policy of the Harvard 

 College Observatory to send expeditions to observe total 

 eclipses of the Sun. First, since in the case of cloudy 

 weather, no return is obtained for the ex23enditure of a 

 sum of moncN' which is often large. Secondly, if clear, 

 the results in many cases are only a series of pictures 

 of the corona and protuberances which add but 'ittle 

 to our knowledge of them. Therefore, when officers of 

 this Observatory have observed eclipses, it has generally 

 been largely at their own expense. When, however, a 

 new problem presents itself some aid is rendered from 

 the funds of the Observatory, in the construction of 

 instruments and for similar expenses. The following 

 plan for obsci-\'ing the Eclipse of May 28th, 1900, has 

 been prepared by Professor W. H. Pickering : — • 



It is a fact capable of demonstration, that the faint- 

 uess of a star that may be photographed with a given 

 instrument, against a bright background of sky dejjends, 

 within certain limits, directly on the length of the focus 

 of the lens, and is independent of its aperture. 



In the Harvard Observatory Annals, Vol. XVJII., 

 p. 104, it was shown that if the place in which to lool 

 for the Pole Star is known, that three minutes after it 

 first becomes visible to the naked eye in the evening, 

 the light of the sky in its immediate vicinity is of about 

 the same photographic intensity as that of the sky sur 

 rounding the Sun at the time of a total solar eclipse. 



Starting with these two fundamental facts, a series oi ex- 

 periments has been undertaken with a photographic lens 

 having an aperture of 3 inches, and a focal length of 

 11 feet 4 inches. The curves adopted were those em- 

 ployed in an ordinary landscape lens, and it was found 

 that the field was large enough to cover nine 8x10 

 photographic plates arranged in three rows of three each 

 This result was only obtained, however, by attaching 

 the plates to the interior of a concave siu-face of double 

 curvature, and thus obtaining a curved field. 



By giving an exposure of one minute in the region of 

 the Pole, with this instrument, three minutes after the 

 Pole Star first became visible, it was found that the 

 light of the sky was sufficient to darken the plate appre- 

 ciably, but not so much as to prevent stars of the eighth 

 magnitude appearing with sufficient intensity to be 

 found by a careful search, in the larger part of the field 

 of view. 



Three similar lenses have now been ordered, and the 

 four will be placed upon one mounting, in such a manner 

 as to photograph a region extending for sixteen degrees 

 on cither side of the Sun, and having a breadth of ten 

 degrees throughout its length. Throughout nineteen 



