3i« 



NATURE 



[February 2, 1893 



The Brazilian Government are willing to place a war 

 vessel at the disposal of the foreign expeditions to 

 observe the eclipse, and it is hoped the English observers 

 will be able to avail themselves of the privilege thus 

 gracefully offered. The station selected is at Para Cura, 

 on the coast about forty miles west of Ceara, and the 

 party will rely upon obtaining any necessary help from 

 the Brazilian authorities and from local assistants. 

 The observers will return from Pernambuco by the Royal 

 Mail steamer due to leave there on April 22, and expect 

 to be in England on May 5. 



The objects of the expeditions are — 



(i) To obtain visual photometric measures of the light 

 of the corona. 



(2) To obtain photographs of the corona with the four- 

 inch lenses of a little over sixty inches focus belonging 

 to Captain Abney, which were successfully used in Egypt 

 (1882), CaroHne Island (1883), Granada (i 836), and Salut 

 Isles (1889), in order to continue the series. 



(3) To obtain enlarged photographs of the corona 

 with small photographic action, so as to show details of 

 the structure of the brightest parts, i.e. those nearest the 

 sun. 



(4) To measure the photographic intensity of the 

 light of the corona, by direct comparison with standard 

 intensity scales placed on the margins of the plates used 

 for the negatives to be obtained under sections 2 and 3. 



(5) To obtain photographs of the spectrum of the 

 corona. These spectra will be obtained on three dif- 

 ferent plans : — 



{a) With integrating spectroscopes, where no colli- 

 mator is used and the prism or prisms are placed directly 

 in front of the object glass of the photographic camera. 



{b) With ordinary slit spectroscopes, the slit being 

 arranged as a radius of the sun. 



{c) With ordinary slit spectroscopes, the slit being 

 arranged as a tangent to the sun's limb. 



The first of these objects will be attempted only at the 

 African station ; Prof. Thorpe and his assistant, Mr. Gray, 

 making the observations. Their equipment wiirconsist of 

 a six-inch Simms equatorial of seventy-eight inches focus 

 (lent from Greenwich) fitted with special photometric ap- 

 paratus lent by Captain Abney. The observations will be 

 ^ made on essentially the same plan as that pursued by Prof. 

 Thorpe at Hog Island, near Granada, in 1886, separate 

 portions of the corona being compared with a standard 

 glow lamp by means of a Bunsen photometer. An inte- 

 grating box for measuring the total coronal light with as 

 little light from the sky as possible, and an ordinary 

 Bunsen's bar photometer will also be used, these being 

 entrusted to officers of the gunboat. 



As regards objects 2, 3, and 4, duplicate apparatus has 

 been arranged for use at the two stations. 



A photoheliograph mounting from Greenwich has 

 been lent for Brazil, and an exactly similar instrument 

 from South Kensington for Africa. On each of these 

 mountings a specially designed new double tube will be 

 fixed. An Abney lens will be mounted in one compart- 

 ment of each of these tubes, and this, with a focal length 

 of sixty inches, will give pictures on the scale of rather 

 more than half an inch to the moon's diameter. In the 

 other compartment a four-inch Dallmeyer photohelio- 

 graph lens will be mounted in combination with a 

 specially-constructed two-and-a-half-inch Dallmeyer 

 negative lens of eight inches negative focus ; this arrange- 

 ment giving, with a total length of sixty-eight inches, 

 pictures on the scale of over one-and-a-half inches to the 

 moon's diameter. This latter arrangement is essentially 

 ' the same as that of Dallmeyer's new telephotographic 

 lens. It will be so arranged that the ratio between the 

 photographic effect of the Abney lens and the new com- 

 bination will be as 10 : i. 



Special plate holders have been made to fit the double 

 tubes, each of these plate holders carrying two plates, 



• NO. 12 J4, VOL. 47] 



which will be exposed simultaneously to the images 

 formed by the Abney lens and the enlarging combination. 

 The six separate exposures, giving twelve photographs, 

 will be so arranged that the longest exposed pictures with 

 the enlarging combination will have received the same 

 photographic action as the shortest exposed pictures with 

 the Abney lens. The whole of the pictures will thus 

 form a continuous series, all the short exposures in the 

 series having a direct enlargement of three diameters. 



In Brazil Mr. Taylor will take charge of this double 

 instrument, and in Africa the similar instrument will be 

 entrusted to Sergeant Kearney. On the night before the 

 eclipse intensity scales for object 4 will be impressed by 

 the use of standard lights and specially-constructed scales 

 kindly supplied by Captain Abney on all the plates to be 

 exposed to the corona. The plates will be developed at the 

 stations as soon as convenient after the eclipse, experience 

 on previous occasions, both by English and American 

 observers, having shown that it is impossible to repack 

 undeveloped plates after exposure in the tropics, and 

 bring them home without serious deterioration. 



Similar spectroscopic work is to be carried out at the 

 two stations. For the integrating spectroscope in Africa 

 Mr. Fowler will use a six-inch objective prism with a six- 

 inch photographic lens of about nine-feet focus, mounted on 

 an equatorial stand, belonging to Prof. J. Norman Lockyer, 

 and kindly lent for the expedition. At the Brazilian 

 station Mr. Shackleton will use two three-inch prisms in 

 front of a three-inch photographic lens of about two-feet 

 focus ; the spectroscope, which belongs to South Kensing- 

 ton, being arranged horizontally and used with a ten-inch 

 heliostat, also lent by the Science and Art Department. 

 Very short exposures will be given at each station at the 

 commencement and end of totality, so as to obtain, if 

 possible, the very numerous bright lines which have been 

 observed in the chromosphere ; and exposures of from 

 5 to 45 seconds will be given during totafity. 



In Africa the radial and tangential slit spectroscopes 

 will be mounted together on the Corbett equatorial 

 stand lent from Greenwich, the spectroscopes used 

 belonging to the Royal Society. Mr. Fowler and 

 Sergeant Kearney will erect and adjust these instru- 

 ments, but the actual exposure, which will extend through 

 the whole of totality, will be made by an officer of the 

 gunboat who will be placed in charge of the instrument. 

 In Brazil the radial and tangential slit spectroscopes will 

 be mounted horizontally and used with a second ten-inch 

 heliostat lent by the Science and Art Department. The 

 erection and adjustment will be made by the observers, 

 but the actual exposure during totality will be entrusted 

 to a local assistant. Orthochromatic plates will be used 

 for all the spectroscopic work, the spectra obtained 

 extending from above D into the ultra-violet. 



Briefly summarised, the English programme is as 

 follows : — 



In Africa :— Prof. T. E. Thorpe, assisted by Mr. Gray 

 and local assistance— Photometric measures of the 

 visual intensity of the corona with the equatorial photo- 

 meter, the integrating photometer, and the bar photo- 

 meter ; Mr. Fowler — The six-inch integrating spectro- 

 scope ; Sergeant Kearney — the Abney and Dallmeyer 

 coronographs ; local assistance — the radial and tangen- 

 tial slit spectroscopes. 



In Brazil : — Mr. Taylor, the Abney and Dallmeyer 

 coronographs ; Mr. Shackleton, the three-inch two-prism 

 integrating spectroscope ; local assistance, the radial and 

 tangential slit spectroscopes. 



It is not yet decided whether one of the 20-inch mirrors 

 of 45-inches focus specially constructed to photograph 

 the faint extensions of the corona during the eclipse of 

 1889 (December 21-22) will be taken to Africa. If 

 so it will be entrusted to a local assistant. It was 

 originally intended to use one of these in Africa, and it 

 was hoped that one would be used by the Harvard College 



