September 9, 1922J 



NA TURE 



357 



tions of the planet Eros, and the determination of 

 photographic magnitudes of stars. 



Behind B, at C, is a drum-shaped dome beneath 

 which Airy's altazimuth for observations of the moon 

 was situated. This instrument was taken down in 

 191 1 and a photoheliograph installed, with which the 

 daily photographs of the sun are taken. These with 

 photographs taken at the Cape and supplemented 

 by others from India give a complete daily record of 

 sunspots. They are measured and the results utilised 

 to determine the peculiarities of the sun's rotation and 

 the remarkable n-year period in sunspots and their 

 connexion with terrestrial magnetism. 



To the east of the astrographic equatorial is the 

 transit circle in a building, D, with a gable roof running 

 north and south. This instrument, erected in 1851 by 

 Airy, is on the Greenwich meridian. It has been in 

 constant use for seventy years to determine Greenwich 

 time, and with it regular observations are made for 

 the positions of sun, moon, planets and stars. These 

 observations have contributed very materially to the 

 foundation of the tables and catalogues from which 

 the " Nautical Almanac " is computed. In addition, 

 many other stars have been observed for comparison 

 with earlier observations made by Bradley and others 

 at Greenwich and elsewhere. The small changes 

 in position of the stars among themselves as seen in 

 the sky determined in this way and combined with 

 other data give the sun's motion in space, the average 

 distances of stars, and prove the existence of two 

 streams of stars. 



To the left is a building, E, surmounted by a small 

 dome containing the Sheepshanks telescope, used for 

 observations of comets. These buildings also contain 

 office rooms, record rooms, a small laboratory for 

 receiving the wireless time signals from Paris, Bordeaux, 

 Lyons, Nauen, Annapolis, and other stations, and 

 rooms in which chronometers and watches are stored. 



The large dome, F, contains the 28-inch equatorial. 

 This large telescope was erected in 1893 on a mounting 

 which had carried the 13-inch Merz telescope, which 

 at the time of its erection in i860 was the largest 

 telescope in the observatory. The 28-inch refractor 

 is used mainly for the measurement of close double 

 stars. 



The care of the chronometers and watches belonging 

 to the Royal Navy is part of the work of the observa- 

 tory, and the room beneath the large telescope is the 

 main chronometer room ; it contains ovens in which 

 chronometers and watches are tested to see that they 

 are correctly compensated. In this room is preserved 

 the beautiful chronometer, made by Harrison, which 

 obtained the Government prize of 2o,oooZ., and a copy 

 of it made by Larcum Kendal which was carried by 



Capt. Cook on his voyage round the world. On the 

 ground floor is another room for chronometers and 

 refrigerating plant for testing them at low temperatures. 



A little to the south is the altazimuth, G. This was 

 erected by Sir William Christie in 1896, and is used to 

 supplement the observations of the moon made with 

 the transit circle. A little south-east is a building used 

 as a store-room, in which the publications of the 

 Greenwich and Cape observatories are housed. 



At the extreme south is the new building erected in 

 1896-1898. It is cruciform, and has on the main floor 

 office rooms, where photographs are measured, calcula- 

 tions made, and observations discussed. The basement 

 consists of libraries and a workshop. Three of the 

 rooms of the upper floor are used for storing records 

 and photographs, the fourth is used for photographic 

 work, such as reproduction of the Franklin Adams 

 Charts. The central part of the building is surmounted 

 by the Thompson equatorial under a 36-ft. dome, H. 

 This instrument, given by Sir Henry Thompson, con- 

 sists of two telescopes on the same equatorial mounting. 

 One of these is a 26-inch photographic telescope with 

 a guiding telescope of 13 inches, and the other a 30-inch 

 reflector made by Dr. Common. The photographic 

 refractor has been put to many uses, including the 

 observation of satellites and minor planets. It is at 

 present used regularly for the determination of the 

 distances of stars, delicate work which is carried on 

 very successfully in spite of difficulties arising from 

 weather and short summer nights. The reflecting 

 telescope has been used for photographing nebulae, 

 comets, and faint objects where great light-grasping 

 power is required. With it a very faint and distant 

 satellite of Jupiter was discovered. It is at present 

 employed with a coarse grating to determine the 

 colours of stars, or with a spectroscope to study the 

 distribution of light in stellar spectra. 



Only one important instrument is not shown in the 

 photograph. The Cookson telescope, lent by the 

 University of Cambridge and used for observation of 

 small movements of the earth's pole, is in a small 

 wooden hut to the east of the octagon room, and is 

 hidden by that building. 



Near the top left-hand corner, I, of the photograph 

 is an enclosure in which are two buildings devoted to 

 magnetic observations. Magnetic instruments had 

 to be moved some distance from the main observatory, 

 owing to the effect of iron in the domes on the magnets. 

 Continuous photographic traces are taken showing 

 the changes of the magnetic elements, controlled by 

 regular observations made by eye. This enclosure 

 also contains a radium collector for the study of 

 atmospheric electricity, and various meteorological 

 instruments. 



Colour Vision and Syntony. 



T 



By Prof. E. H. 

 HE classic theory of colour vision due to Young 

 and developed by Helmholtz and Maxwell at- 

 tributes the observed phenomena to three sensations 

 (red, green, and violet), but does not enter into detail 

 as to the type of mechanism involved in their stimulus 

 and response. In the days before the discovery of the 

 electron, it was unlikely that any hypothesis of syntony 



NO. 2758, VOL. I 10] 



Barton, F.R.S. 

 (ar sympathetic vibratory response) should be de- 

 veloped for the eye, because nothing was then known 

 of vibrators capable of such high frequencies as those 

 involved in the visible spectrum. But it is now open 

 to us to attempt a syntonic hypothesis of colour vision, 

 since so much more is known as to the constitution 

 of the atom and the behaviour of the electrons. The 



