REPORT OF THK KEW COMMITTEE. XXXV 



meters, 45 barometers. There have besides been verified for opticians 

 92 thermometers. The total number of instruments verified up to this time 

 is 2032 thermometers, 155 barometers, 800 hydrometers. 



The Chairman has received an application, through Colonel Sabine, from 

 Dr. Pegado, Superintendent of the Royal Marine Meteorological Observatory 

 at Lisbon, for a Kew Standard Thermometer, and for specimens of the Marine 

 Barometers, Thermometers and Hydrometers, supplied to the British Navy 

 and Board of Trade, accompanied by an inquiry whether a supply of such in- 

 struments can be obtained for the Portuguese Royal Marine by the aid of the 

 Kew Committee of the British Association, the centesimal scale being em- 

 ployed in the thermometers, and the metrical scale in the barometers. The 

 instruments thus applied for are in course of preparation, and the Kew Com- 

 mittee signified to Dr. Pegado their readiness to undertake the verification 

 of Marine Meteorological Instruments for the Portuguese Government (if 

 desired), under similar arrangements to those which have been approved and 

 adopted by our own Government and by the Government of the United States. 



The increased demand on the time and work necessary for the verifica- 

 tion of instruments in the Observatory, renders it necessary for the Committee 

 to employ further assistance. As yet the Committee have not been able to 

 obtain the permanent services of any person of the character they require ; 

 ^ut in the meantime, Dr. Hermann Halleur, of Berlin, at a weekly salary of 

 30s., on the recommendation of Colonel Sykes, has undertaken for a short 

 time to assist Mr. Welsh in the verification of the instruments. 



The Committee has caused a room for magnetic experiments to be erected 

 in the ground, at a cost of about £50. 



The apparatus suggested by Sir John Herschel for photographing 

 the spots on the sun's disc, is progressing under the superintendence of 

 Mr. Warren De la Rue. The Solar Photographic Telescope is promised by 

 the maker complete in three months ; the object-glass is finished, and some 

 progress has been made with the stand. The diameter of the object-glass is 

 3'4 inches, and its focal length 50 inches ; the image of the sun will be 

 0-465 inch, but the proposed eye-piece will, with a magnifying power of 

 25*8 times and focal length x, increase the image to 12 inches, the angle of 

 the picture being aboui 13° 45'. The object-glass is under-corrected in such 

 a manner as to produce the best practical coincidence of the chemical and 

 visual foci*. The eye-piece consists of two nearly achromatic combinations, 

 their forms, foci, and focal lengths being arranged upon the basis of the 

 photographic portrait lens, the conditions being nearly similar. 



It is contemplated to form the system of micrometer-wires on a curved 

 surface ; and it may ultimately be found to be advantageous also to curve 

 the photographic screen, as the small curvature necessary, namely about 

 two-tenths of an inch, will present no mechanical difficulties. As in practice 

 it may possibly be found desirable not to produce the sun's image with too 

 great rapidity, a provision is contemplated for the absorption of some of the 

 most energetic active rays by the interposition of coloured media of different 

 tints. 



The telescope being for a special object, it will have no appliances except 

 such as appertain exclusively to that object, so that the only means provided 

 for viewing the sun will be through the finder intended for facilitating the 

 adjustment of the sun's image in position as regards the micrometer. The 



* Mr. Ross has found, that if for the greatest intensity of vision, in common lenses, the 

 ratio of the dispersive powers of the two media is 0-65, the chemical and visual foci 

 will coincide best practically when with the same media the ratio is altered to 0*60 ; the 

 media he sometimes uses being Pellatt's flint and Thames plate. 



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