26o 



NATURE 



[December 30, 1909 



published by the International Latitude Service since 1908. 

 Periods of one-fortieth of a year (9-125 days) were taken, 

 and the values for x. and y analysed independently, special 

 attention being paid to the possible demonstration of a 

 vearly period. 



The resulting maxima show no trace of an annual 

 period, but there is a weak maximum at thirteen months. 

 A very strongly marked period, of 419-750 days for the x, 

 and 410-625 days for the y, coordinate, was found, and 

 agrees fairly well with the period (42S days) found by 

 ■Chandler. Another less marked period of 438-0 days also 

 results from M. Krassowski's investigation, and agrees 

 with that found by Mr. Kimura. 



The Planet Vexus. — In a very interesting lecture, now 

 published in the December number of the Popular Science 

 Monthly (vol. Ixxv., No. 6, p. 521), Prof. Lowell describes 

 the observations which have been made of the planet Venus, 

 at present so prominent an object in our evening skies. 

 Not only are the observations described, but the results 

 accruing from them are discussed in popular language. 

 Thus the spectroscopic and visual observations are held to 

 have proved that tlie rotation and revolution of Venus are 

 synchronous, the period being 225 days. A number of 

 drawings accompanying the paper show the permanent 

 markings recorded, and illustrate the unanimity of the 

 observers working under good conditions. The main 

 feature is a number of dark markings whicli, leaving the 

 limb at different points, converge to the centre, thus giving 

 the planet's disc a cart-wheel appearance. 



SUGGESTED OBSERVATIONS OF HALLEY'S 



COMET. 

 'T'HE Astronomical and Astrophysical Society of America, 

 through its comet committee, is soliciting coopera- 

 tion in the observation of Halley's comet at the present 

 return, and has prepared a circular letter of advice that 

 has been widely distributed among observatories with re- 

 gard to such observations. A copy of this circular will be 

 sent to any astronomer who may desire to use it upon 

 request being made to the chairman of the committee. 

 Prof. G. C. Comstoclc, Washburn Observatory, Madison, 

 Wisconsin. As many astronomers and other observers of 

 Halley's comet will be interested in the suggestion made 

 \>y the committee, the circular is here reprinted in a slightly 

 abridged form. 



It is desirable that the position of the comet be well 

 •observed during the entire period of its visibility, and it 

 seems probable that extra-meridian observations will be 

 secured in sufficient number without especial solicitation. 

 In view, however, of possible large perturbations arising 

 from the close approach of the comet to Venus on May 1, 

 and to the earth on May 18, meridian observations are 

 especially desired during the period in which the comet is 

 sufficiently bright for that purpose. An examination of the 

 amount and character of these comet perturbations and 

 their adaptability to a determination of the mass of the 

 planets producing them has been undertaken by Profs. 

 Leuschner and Crawford, and in case the conditions prove 

 favourable, the meridian determinations may well be sup- 

 plemented by heliometer observations of the positions of 

 the inner planets with the view of a possible determination 

 of the mass of the comet itself. 



The close approach of the comet to the earth promises 

 unusual opportunity for a study of the physical conditions 

 that obtain in such a body, and, as an indispensable basis 

 for such study, the committee recommends a photographic 

 campaign as long and as nearly continuous as possible. 

 The comet's close proximity to the sun's direction at the 

 time of maximum brilliance imposes serious limitations 

 upon this programme, and widely extended cooperation 

 will be required throughout the whole circuit of the earth 

 if this ideal of a continuous photographic record is to be 

 even remotely realised. 



About one-third of the earth's circumference in longitude 

 is covered by the Pacific Ocean, within which there is 

 known to exist no observatory with proper facilities for 

 celestial photography. To fill this gap, at least partially, 

 the committee, aided by a grant from the National Academy 

 KG. 2096, VOL. 82] 



of Sciences, proposes to send to the Hawaiian Islands an 

 expedition to photograph the comet during the period of 

 its greatest brilliance. 



The ends to be served by these photographs, and others 

 obtained elsewhere, are as follows : — 



To give a permanent record, as continuous as possible, 

 of the phenomena and changes (i) in the tail of the comet, 

 with special reference to outgoing masses; (2) in the head 

 and nucleus of the comet, particularly as to the formation 

 of envelopes and jets. 



The following suggestions as to procedure and precaution 

 in making the photographs have been formulated by Prof. 

 Barnard. 



Photography of Comets. 



One of the greatest difficulties in photographing the 

 average bright comet is its proximity to the horizon, and 

 consequent projection on a more or less dawn or twilight 

 sky. The effect of this illuminated background with any 

 considerable exposure is to fog the plate to such an extent 

 as either to ruin it or to prevent a proper development of 

 the image of the comet. A difference of three or four 

 minutes in the duration of exposure when the sky is 

 brightest may make a success or a failure of the picture. 

 It is impossible to establish fixed rules as to when the 

 exposure should stop or begin ; so much will depend upon 

 the condition of the sky, the position of the comet, th'i 

 kind of lens, the rapidity of the plates, &c. The best rule 

 is that of the judgment of the observer at the time, and 

 this can only be derived from actual experience in the 

 work. 



The plates should be backed with the following to 

 prevent halation. Cook two pounds of white sugar in a 

 saucepan without water until nearly in the caramel stage, 

 then add one pound of burnt sienna. Cook a little more 

 (but not to the candy stage), stirring well. Finally, add 

 about one-half an ounce of alcohol to each pint of backing 

 as a dryer. This backing will keep indefinitely. When it is 

 too hard, moisten it with a little water. This is to be 

 applied to the back of the plate as a stiff paste with a 

 broad camel's-hair brush, and should be applied just before 

 using. A piece of old newspaper pressed upon this will 

 prevent its being rubbed. The face of the plate should 

 be very carefully dusted with a broad camel's-hair bru;h 

 after it has been placed in the plate-holder. The camera 

 tube should also be frequently wiped out with a damp 

 cloth to avoid dust. Before developing, remove the back- 

 ing with moist absorbent cotton. If a little remains on 

 Ihe plate it will not injure the developer. In removing 

 the backing be careful to shield the plate from the dark- 

 room light. Do not wet the surface of the plate before 

 pouring on the developer, as it may cause air bubbles on 

 the film ; swab it carefully with absorbent cotton at the 

 beginning of development. Develop until the plate is 

 almost opaque to the ordinary developing light. Fix for 

 twenty minutes or more in the ordinary fixing bath (fre- 

 quently made new), to which has been added a teaspoonful 

 of sodium bisulphite to prevent discoloration. 



Lumifere Sigma dry plates are recommended, because of 

 their rapidity. Seed 27 Gilt Edge and Cramer Crown are 

 both beautiful plates, but are not now so rapid as the 

 Sigma. 



Hydrochinon developer gives a good strong negative, and 

 for astronomical work is excellent. Rodinal in a weakened 

 form, say_ 1/60 or 1/70 of water, with a longer develop- 

 ment, will give a soft and more transparent negative, 

 especially suited for showing the details of the head of the 

 comet on large-scale photographs. 



The doublet, or portrait lens, such as is made in America 

 by the Brashear Optical Company and the Alvan Clark 

 Corporation, on account of its wide field, is the best form 

 of instrument for showing the general features of the 

 comet and its tail, and especially for following any out- 

 going masses that mav appear in the tail. One of about 

 6 inches (15 cm.) aperture will be the most generally used, 

 because of the expense of such instruments. It should be 

 supplemented by several smaller lenses. A " lantern " lens 

 of li inches (4 cm.) aperture and about 6 inches (15 cm.) 

 focus, made by McAllister, of New York, is recommended 

 for showing the extent of the tail. The cost of one of 

 these lantern lenses is seven dollars. It gives a good field 

 of twenty to thirty degrees, especially when diaphragmed 



