176 



KNOWLEDGE 



[August 1, 1900. 



of articles. I propose, therefore, on the present occasion 

 simply to catalogue the results which seem to me to be 

 of most importance. 



1. Large Scale Photographs.^ — By large scale photo- 

 graphs I mean photographs giving a diameter of four 

 inches or more to the moon's disc. These are becoming 

 more and more a regulai' feature of eclipse work, and 

 on the present occasion both the Astronomer-Koyal and 

 the Astronomer-Royal for Scotland from this country 

 undertook this department with great success. The 

 instruments were of very different types. The Astronomer- 

 Roy als camera possessed an object glass of 9 inches 

 apertirre and only 8i feet focal length, a four inch image 

 being obtained by a negative combination within the 

 primary focus ; the camera was fixed and fed by a 

 ccelostat. Dr. Copeland's insti-ument was the 40 foot 

 focus lens which he took to Norway and to India. This 

 was not pointed direct to the sun, as at Vadsb in 1896, 

 but the light was reflected into it by a fixed mirror, and 

 the plat-e was made to travel instead of the telescope. 

 This ample scale has been exceeded by the American 

 astronomers, who have used object glasses of 61i and 

 133 feet focus, securing photographs on scales of seven 

 and fifteen inches to the lunar disc. 



Without dwelling at length upon the beautiful detail 

 both of corona and prominences shown on the Astro- 

 nomer-Royal's photographs, a comparison of his Indian 

 and Portuguese negatives teaches a very significant 

 lesson. Valuable as each series is in itself, it is not too 

 much to say that each has a double value in its compari- 

 son with the other. It is most earnestly to be hoped that 

 no slight difiiculty will be allowed to prevent a series so 

 magnificently begun being continued, eclipse after 

 eclipse, with the same instrument and on the same 

 scale. The closing in towards the equator of the great 

 extensions, the diminution of structiu'e in the lower 

 corona, the greater separation of the polar plumes, and 

 the greater amount of general diffused, amorphous 

 coronal light, as seen in the Eclipse of 1900 when com- 

 pared with that of 1898, is most evident. This vear's 



Fig. 1. — The Meteorological Instruments and Shadow-Band Sheet, 

 Hot«l de la Kegence, Algiers. 



Fioni a Photo by Miss Edith MLvundek, 



eclipse was emphatically an eclipse of the sun-spot 

 minimum; it reproduced the general form, — it is 

 scarcely an exaggeration to say, even the detail, — of the 

 Eclipses of 1878 and 1889, at the two preceding minima 

 with astonishing fidelity. 



2. Medium Scale Photographs; that is to say, of a 



seals of half-an-inch to two inches. These were too 

 numerous to catalogue, but here we must express a 

 regret. For a long series of years the British ofiicial 

 expeditions have taken photographs with identical lenses 

 of about 5 feet focus. It is a pity that the series has 

 this year been brought to a close or at least interrupted. 



3. Small Scale Photographs; that is to say, less 

 than half-an-inch in diameter. A large number of these 

 were no doubt taken with fixed cameras in consequence 

 of our having pointed out that for most purposes there 

 was no need to use a driving clock with short focus 

 lenses. But several were taken directly in consequence 

 of the success in their delineation of the coronal ex- 

 tensions of our long exposure photographs in India. 

 The result of these so far as we have yet heard has been 

 to show distinctly that it was not possible in this eclipse 

 to photograph the streamers to the same extent as in 

 1898. but, on the other hand, quite a short exposure 

 proved practically as effective as the most lengthy given, 

 in bringing them up. Still the character of the ex- 

 tensions was the same; the typical coronal curves 

 running as in 1898 into rod-like rays. 



Before leaving the photographs of the corona, it may 

 be worth while to mention a mistake into which 

 apparently more than one photographer has fallen, that 

 of driving on the moon instead of on the sun. A 

 stationary camera will give a blurring of 15" of arc for 

 an exposure of one second of time ; one made to follow 

 the moon gives a blurring of h" in arc for an exposure of 

 the same time, and vice i-ersa if it follows the sun, the 

 blurring of ths moon's limb in the direction of motion 

 will be of the same amount in the maximum. An 

 exposure therefore of | of a minute would mean a 

 blurring of considerably more than a third of a minute 

 of arc. This would mean 1/100 inch for an inch sun, or 

 a millimetre for one of 4 inches. These are very con- 

 siderable amounts, hence a long exposure photograph 

 cannot be given so as to ensure sharpness both of moon 

 and of corona. A sharp moon under such circumstances 

 means a blurred corona. 



4. Integrating Photographs. — Several Members of 

 the British Astronomical Association, myself amongst 

 the number, devised a method for exposing photographic 

 plates to the general light of the corona in 1896. The 

 unfortunate weather on that occasion prevented the 

 scheme being carried out. but Mr. Gare and Mr. A. H. 

 Johnston arranged a careful scheme in 1898, the execu- 

 tion of which was successfully carried out by Mr. E. 'W. 

 Johnson. The same observers repeated their experi- 

 ments at Manzanares and Elche in Spain this year, and 

 they have had a follower in Professor H. H. Turner, 

 who carried out a similar work at Algiers. Professor 

 Turner's result shows this eclipse to have been very 

 considerably brighter than the Indian ; Mr. Gare finding 

 the corona seven times as bright as the moon in 1898, 

 Professor Turner putting it at ten times the moon 

 in 1900. 



5. Standardized Photographs. — Most, if not all of 

 the photographs of the corona obtained by the British 

 official observers have been " standardized " by the im- 

 printing upon the plates of a series of squares represent- 

 ing known light values. This practice gives to the 

 photographs an entirely new importance over and above 

 the value they possess as pictures, and it is much to be 

 wished that the practice were more general with inde- 

 pendent observers. The work of measuring and reducing 

 the photogi-aphs of the late eclipse can scarcely have 

 proceeded very far as yet. and no results in this line 

 have yet appeared from them; but similar results from 



