124 



KNOWLEDGE & SCIENTIFIC NEWS. 



[June, 1904. 



The adaptation of the Forty-Inch Visual 

 Refractor of the Yerkes Observatory 

 to Photography. 



In till' original design of the forty-inch refractor of the 

 Verlies ( )bservatory, no provision of any Uind was made for 

 direct photography ; there is no gniding telescope to enalile 

 lengthened exposnres to be given, nor photographic corrector 

 to firing the actinic rays to a focns on the sensitive plate. 

 Mi. (i. 'I'. Ritchey has overcome the first difficnlty by means 

 of an eyepiece magnifying abont one thons.uid diameters, 

 placid in the side of a donble slide carrier. A small diagonal 

 prism receives the light of the gniding star, and refiects it at 

 right angles into the eyepiece, and this with its accessories 

 are monnt<'d on a slide which can be moved to any desired 

 position on tlie upper side of the rectangular bo.x, and firmly 

 clamped there, so as to assist in finding a suitable guiding 

 star. The star is brought to the intersection of the cross-lines 

 in the ejepiece, and is Uept there throughout the exposure of 

 the sens live plate. The observer sits with his eye at the 

 gniding eyepiece and his fingers on the two screws which 

 move the slides, and thus he introduces any minute correc- 

 tions of position which he sees are necessary. These correc- 

 tions may be on account of either the irregular movementsof 

 the driving clock of tlie telescope, or more frequently from the 

 tremors in the atmosphere. The latter irregularity may 

 reipiire correction several hundred times in a minute, and a 

 practis-'d ob.=erver can introduce between one and two 

 hundred per miunte. The other difficulty — that the instru- 

 ment is a visual one — Mr. Kitchey has obviated by the n.se of 

 a delicately tinted yellow screen. This screen utilises the 

 ravs of light which are most freely transmitted by a large 

 objective; since it is a well Unown fact that while only a 

 small percentage of the yellow rays are lost by transmission 

 throirih a Large and necessarily thick ol)jecti\e, a veiy large 

 percentage of the blue rays are. Consequently the forty-inch 

 visual objective, thus used with a yellow screen, and plates 

 sensitised to the yellow rays, is scarcely less rapid, if at all, 

 ni photographing stellar images, than an ofiject-glass cor- 

 rected for blue rays would be. In two hours it registers stars 

 of approximately the seventeenth magnitude, which are at 

 the visual Hunt of the instrument ; and in five hours can 

 register stars of a magnitude fainter. The yellow screen is 

 formed from two tliin and transparent plates, finely ground 

 flat and highly polished. One of these plates, wliich are S by 

 10 inches, is flowed over with a collodion film of a delicate 

 yellow tint, and when the film is dry, this is covered with 

 Canada balsam, and the other plate bound on it as a cover 

 glass by adhesive tape. When in use it is laid close upon 

 the sensitive plate, nothing separating them but the tape. 

 Mr. Ritchey has been most successful in photographing por- 

 tions of the moon's surface, and close clusters of stars, and 

 in Vol. VHI. of the Decennial Publications of the University 

 of Chicago, several very fine specimens are given, notably one 

 of the lunar cnitcr Theophilus and its surroundings, which 

 perhaps shows the detail on the moon's surface more clearly 

 than any otlicr photograph ever taken. In the photographs 

 of the clusters Messier 13 and 15, the original neg.itives and 

 transparencies from them show the star images separ.ite and 

 distinct, even at the very centre of the cluster, but in the pro- 

 cess reproductions given in the volume the smaller and nearer 

 stars are mi^rgcd together. With nebula; the yellow screen is 

 not so successful since these are lich in their proportion of 

 green rays, which do not come to the same focus as the 

 vellow. 



■«• * * 



Photographs with the Two-Foot Reflector 

 of the Yerkes Observatory. 



Seven very fine specimens of Ihc woi k done with the two- 

 foot reflector of the Yerkes ( )bservatory are published in 

 Vol. VIII. of the Decennial I'nblications of the University of 

 Chicago. These are of the two giant nebula- of Orion and 

 Andromeda ; of the spiral nebuhe Messier ;5 Tiiangnli and 

 Messier 51 Canum Venaticoriin ; of the c.irded-wool-like 

 nebulosity in the Pleiades; and of tlie torch-like nebuhe in 

 Cygnus known as N.G.C. 6960, and N.G.C. ht)i)>. These two 

 last form part of the same extended nebulosity, but they 



present some striking differences in their relationship to the 

 stars. In the first ease the nebula seems to act as a wall or 

 barrier separating a region strewn very thickly with stars, 

 from a sparser field ; in the other case no such difference in 

 the numlier of the stars seems to exist on the two sides of the 

 nebul.i, which itself appears to lie in a district of few and small 

 stars. 



'i^<ti^t^(ti 



ZOOLOGICAL. 



Mosquitoes in England. 



Di'Siu rr; the coldness and wetness of the season, mosquitoes, 

 according to the " Report on Economic Zoology," issued by 

 the Trustees ot the f-Sritish Museum, .appear to have been 

 unusually numerous in England last summer, and to have 

 caused much annoyance and inconvenience. They were 

 ■\cry prevalent in parts of b^ssex, especially in the neighbour- 

 hood of hipping I'orest, and also in Kent and Surrey, notably ■ 

 along the \allcys of the Thames and the Kennet, and in the 

 marshes bordering the lower courses of the Thames and the 

 Lea. They were also reported as having caused much annoy- 

 ance near Bristol, at Great Staughton, Huntingdonshire, and at 

 Weston-super-Mare, Worplesdon, Colchester, Canterbury, 

 and Birchington. Although complaints of mosquito bite are 

 received almost yearly from the Thames Valley, last summer 

 the in.sects in question .seem to have been unusually virulent, 

 causing such swellings that medical attendance was in soiiie 

 instances recjuisitioned. The species most abundant were the 

 conmion gnat [Cithx pifticm) and the banded gnat {Tlicubalilin 

 auniiUitii), the latter of which does not usually attack man. This 

 reminds us that we fail to see the reason for dropping the 

 good old English word "gnat" in fa\our of the foreign 

 '" uioS(]uito," now th.il both are known to be the same. 



■X- * -X- 



A Deer-like Antelope. 



Hitheito there has been supposed to exist a sharp distinc- 

 tion between deer and .antelopes, according to the nature of 

 their horns ; but recent discoveries iu North .America tend to 

 show that this distinction is only a fcatnre of the present day. 

 Deer, it is almost superfluous to mention, have deciduous 

 bony antlers, while in antelopes the horns are covered with 

 hollow sheaths, which are never shed and never branched. 

 The .American prongbuck resembles antelopes in its skeleton, 

 but its horns are forked. The new fossil type combines the 

 skeleton and teeth of an antelope with the antlers of a deer. 



New British Mouse. 



According to a note by Mr. W. IC. Cl.irkc inthi Proceedings 

 of the Royal Physical Society of Edinburgh, the mou.se of the 

 l-";croe Islands is a large and stouter built animal than the 

 rommoii house mouse, from which it also differs in colour. 

 It is therefore regarded as representing a distinct local race 

 of that species. St. Kilda has also a pecnli.ir mouse of its own. 



A Rare Bird at the Zoo. 



The Zoological Society's menagerie in the Regent's Park 

 has recently received an interesting and valuafile addition in 

 the form of a specimen of the South American boat-billed 

 stork {Citiuhroiiui cochhnr'ui). It is many years since this 

 species, which, by the way, must not be confounded with the 

 shoe-bill of the White Nile, has been represented in the 

 collection. 



•>;■*» 



New Egyptia-n Fossils. 



(ircal interest .attaches to the dcscri|ition by Dr. Fra.is, of 

 Stuttgart, of certain very remarkable fossil niammaliau 

 remains from Lower Tertiary marine strata in the Mokattam 

 range, near Cairo. These specimens serve to show that a 

 gigantic Tertiary whale-like creature, known as Zcui^kniun 

 (of which the remains were first discovered in North America), 

 is the direct descendant of the primitive land Carnivora of 



