April 1, 1899.] 



KNOWLEDGE. 



87 



for surveys made in a short time and repeated at frequent 

 intervals. Such surveys are useful for several purposes. 

 The detection of new planets, of variable stars and Nov.t, 

 can only be satisfactorily carried out by schemes of which 

 such frequent surveys form an integral part. Prof. 

 Pickering, therefore, early devised an ingenious method by 

 which the same plate might be exposed to many different 

 parts of the sky and record the stars in each region in so 

 distinctive a manner that no confusion need take place 

 between the difterent images from each. 



There are two fields of work in which the necessity for 

 embracing a large area of the sky upon a single plate is 

 even greater than in that of the search for variables or 

 NovjB, and yet in which the device of repeated exposures 

 would be absolutely useless. These are the two depart- 

 ments of the study of general stellar distribution and of 

 meteor observation. 



The former subject was one in which Mrs. Maunder took 

 a great interest. It was apparent to her that fields of five 

 or ten degrees in breadth were insufficient for the purpose 

 of dealing with some of the problems of sidereal architec- 

 ture. Regions much wider must be brought together upon 

 a single plate, and that, too, without serious distortion, if 

 the work was to be carried out in the most effective man- 

 ner. There did not seem to be any lens in the market 

 meeting this requirement until Mr. Dallmeyer, whose work 

 as a photographic optician is so well known, brought out 

 his stigmatic lens, covering an unusually wide field. 

 Consequently she applied to Mr. Dallmeyer for one of his 

 lenses immediately upon their details being published, that 

 is to say, early in 1897. It was with this lens — one and 

 a half inches aperture and nine inches focal length — that 

 the photographs were taken of the solar eclipse of January 

 last year, which appeared in Knowledge for May, 1898. 

 The lens was, however, not procured for eclipse work, but 

 for the charting of wide sky areas ; its employment in 

 eclipse work was incidental only, and a portrait lens would 

 probably have been more effective, since a considerably 

 higher ratio of aperture to focus might have been 

 obtained. 



The accompanying plate may serve to illustrate two of 

 the uses to which a lens of this kind might be put. The 

 field shown is thirty-seven degrees in side, fifty degrees 

 in diagonal, and includes one-thirtieth of the afea of the 

 entire sphere. The plate was chosen for reproduction 

 quite at random, and is far from being the best avail- 

 able. Nevertheless it cannot, I think, be in the least con- 

 tested that it shows the largest field, by much the largest 

 field, of stars defined throughout that has ever yet been 

 published. 



The part of the sky depicted is at once apparent, being 

 that which has its centre about 5h. 55m. of R.A. and 8^ 

 N. Dec, and which includes nearly the whole of the 

 constellation of Orion. Stars are seen down to the 

 10th magnitude, and the nebula round 9 Orionis is, of 

 course strongly marked ; the much fainter nebula nf. 

 J Orionis is also very distinctly seen. Two meteors of 

 sufficient brightness to leave a trace fell during the exposure 

 of the plate. One is at the extreme north-west corner 

 of the plate, and only part of the trace is seen. The 

 other is very distinct, and crosses the 17th parallel of 

 N. Dec. about E.A. 5h. 30m. 



Considerably more than a year later than Mrs. Maunder's 

 first employment of this wide-angle lens. Prof. Pickering 

 used a similar lens, as described in Knowledge for 

 September, 1898, for the purpose of the detection of short 

 period variables. Circular No. 40, from the Harvard 

 College Observatory, suggests the use of such lenses 

 for meteor observation, and the photograph at 



once demonstrates its suitability for such work. 

 The mode of operation suggested by Prof. Pickering, 

 and which will shortly be in operation at Blue Hill 

 and at Cambridge, is as follows :— At each station a 

 camera will be exposed to the zenith, each being provided 

 with caps which will close automatically shortly before 

 dawn. About one-third of all the meteors having long 

 paths pass within thirty degrees of the zenith, and all of 

 these, if bright, can thus be photographed. Bright meteor 

 tracks often show points of increased brightness due to 

 small explosions, such as are very noticeable in the 

 meteor in the plate. Such a trail, if photographed at two 

 stations, would give the height of the meteor at the 

 moment of explosion. If the cameras were mounted 

 equatorially, their radiant point could be fixed with great 

 precision. Prof. Pickering goes on to suggest that the 

 use of a prism in front of the lens would secure the 

 spectra of bright meteors, and adds that by the expendi- 

 ture of three plates a night it seems possible to determine 

 the altitude, radiant point, velocity, and spectrum of one- 

 third of all the bright meteors visible in a given locality. 

 It is probable that several meteors bright enough to be 

 photographed in this way appear every month. 



To sum up, then, this new departure in stellar photo- 

 graphy offers the following advantages : — Considered 

 merely as photographs of the heavens, a set of forty plates 

 of the same angular field as the one here shown, but of 

 larger scale, would provide a more complete map of the 

 heavens than any we yet possess, in exceedingly small com- 

 pass, and at very little trouble or expense. The scrutiny of 

 such a set of plates would give us the fullest possible 

 development of that method of attack of the great sidereal 

 problem which Herschel first attempted in his gauges. 

 Indeed, a vast amount of information as to sidereal 

 structure would result from the mere inspection of such 

 plates. Their value in the work of discovery is most 

 obvious. The systematic employment of wide field plates 

 at a number of observatories would ensure the record 

 of all asteroids, comets and Novte within the limits of 

 magnitude recorded ; whilst the ingenious methods which 

 Prof. Pickering has devised will bring about nothing 

 short of a revolution in the discovery of short period 

 variables and the observation of meteors. Much of this 

 work, too, can be done with lenses of small size and 

 therefore little cost, and so lies quite within the powers of 

 many owners of small observatories. For instance, two 

 friends situated north and south of the equator respectively, 

 might easily and at a trifling expense, bring out not only 

 a complete chart of the heavens, embracing over a million 

 stars, but might bring out a new edition every year. 



THE KARKINOKOSM, OR WORLD OF 

 CRUSTACEA.-VIII. 



By the Rev. Thomas E. R. Stebbing, m.a., f.r.s., f.l.s., 

 F.Z.S., Autlior of "■A History of Crustacea," '^ The 

 Naturalist of Cumbrae," ''Report on the Ainphipoda 

 collected by H.M.S. ' Challenyer,' " etc. 



THE DOOR-SHELL CRUSTACEA, or 

 RINGLET-FEET. 



THE sailors who moored their boat to a whale's 

 back, mistaking it for an island, were abruptly un- 

 deceived when they proceeded to light a fire upon 

 it. Cirripedes of sorts treat the massive beast with 

 no more ceremony than the sailors did, and take 

 liberties with better success. They land upon it and embed 

 themselves either at the surface or within the whole thick- 



