December 14, 1899] 



NA TURE 



165 



ellipses. It is possible, therefore, to write down in terms of 

 the angle which one of these planes makes with the axis, the 

 convergivity which would be impressed by the lens upon plane 

 waves travelling in these planes. The effect of a second lens 

 crossing the first obliquely can also be written down with re- 

 spect to the same two planes. The joint effect of the two 

 lenses can then be resolved along any two lines at right angles. 

 Differentiating the expressions for these effects and equating to 

 zero, we get the directions of maximum and minimum cylin- 

 (Iricity. These directions are at right angles, and represent 

 two lenses crossed rectangularly, which are the optical equiva- 

 lent of the original pair considered. The sphero-cylindrical 

 lens is then easily obtained. From the mathematical expres- 

 sions Prof. Thompson has deduced a graphical solution of the 

 problem. The author exhibited a convenient combination of 

 two cylindrical lenses for giving varying degrees of cylindricity. 

 Let two lenses be ground, each being a mixed equi-cylinder 

 consisting of a concave and convex ground at right angles to 

 one another on the opposite faces of the glass. Two such 

 mixed cylinders, if rotated with equal motion in opposite 

 directions, will give a varying cylindricity of fixed direction in 

 space. With the axes of positive cylindricity coincident they 

 !L;ive the maximum ; but when each is rotated 45°, their resultant 

 is zero. When rotated beyond 45°, the resultant axis of cylin- 

 dricity is negative in the fixed direction in which it was formerly 

 positive. — Mr. T. H. Blakesley read a paper on exact formuire 

 for lenses. In this paper the author makes use of the definition 

 of focal length with respect to magnifying power, which he has 

 described in the Proceedings of the Physical Society for November 

 1897. By this method the focal length of a lens combination 

 is simply a line and not the distance between two definite 

 points. Following the methods of his previous paper, Mr. 

 Blakesley shovved how it was possible to determine accurately 

 the constants of lens combinations, and pointed out practical 

 applications to the racking of telescopes for camera work, the 

 determination of refractive indices of liquids, &c. — Prof. W. E. 

 Dalby exhibited a friction dynamometer. The torque to be 

 measured produces a twist in a spiral spring, and the object 

 is to determine the amount of this twist. Side by side upon 

 the shaft are two pulleys, one keyed to the shaft and the other 

 fastened to the end of the spring. The lead of one pulley upon 

 the other, therefore, measures the twist. Two other pulleys 

 are mounted upon a slide, and are joined up to the first ones 

 by means of a continuous band similar to a Weston's differential 

 pulley block. When the shaft is at rest, the two pulleys on 

 the slide are touching ; but any motion of the sha't produces 

 a twist in the spring, and therefore a lead of one of the shaft 

 pulleys on the other. This produces a separation of the slide 

 pulleys, which is proportional to the lead, and therefore to the 

 torque, and so from a knowledge of the constants of the dynamo- 

 meter and its number of revolutions per second the power 

 transmitted is at once determined. — Prof. S. P. Thompson read 

 a note on an organic compound of great double refraction. 

 This substance is crystallised naphthaline, and it is 60 per cent, 

 more doubly refracting than Iceland spar. It is exceedingly 

 brittle, and therefore difficult to work into prisms. Any worked 

 surface must be at once covered with glass to prevent sublima- 

 tion. — The Society then adjourned until January 26, 1900. 



Zoological Society, November 28. — Dr. Henry Wood- 

 ward, F.R.S , Vice-President, in the chair.— Mr. Lydekker ex- 

 hibited (on behalf of Messrs. Rowland Ward, Ltd.) and re- 

 marked on a headless skin of a kob-like antelope from Lake 

 Mweru, which he proposed to call Cobus smithemani, after its 

 discoverer, Mr. F. Smitheman. He likewise exhibited the 

 skull and horns of another kob, belonging to Sir E. G. Loder, 

 for which the name C. vardoni lodcri was suggested. — Mr. Old- 

 field Thomas exhibited the skull of a baboon recently obtained 

 at Aden by Messrs. Percival and Dodson. It appeared to repre- 

 sent a new species allied to Papio hamadryas, but distinguished 

 by its small size, the row of upper cheek-teeth being only 41 5 

 mm. in length. It was proposed to be named Papio arabicus. 

 — Mr. W. Saville-Kent exhibited, with the aid of the lantern, 

 a series of slides demonstrating the utility of trichromatic photo- 

 graphy as applied to the correct colour-registration of biological 

 subjects. Photographic transparencies representing various 

 species of plants and animals were included in the series — Mr. 

 J. S. Budgett gave a general account, illustrated with lantern- 

 slides, of his recent expedition to the Gambia Colony and Pro- 

 tectorate, undertaken primarily for the study of the habits of 

 PolypUnis. Some living and spirit specimens of this fish were 



NO. 1572, VOL. 61] 



exhibited, and remarks were made upon it, as also upon Pro- 

 topterus, of which examples were likewise obtained. Special 

 reference was made to the antelopes met with during a trip up 

 the Gambia River to the end of its navigable waters, and speci- 

 mens of the heads of those obtained were laid on the table. A 

 collection of Gambian birds was also exhibited. — A communi- 

 cation was read from Mr. L. A. Borradaile, in which it was 

 shown that both genera {Coenobita and Birgiis) of the Pagurine 

 \zxvA-cx2^i% {^Coenobitidae) are hatched in the Z<?a^rt-stage. — Dr. 

 W. G. Ridewood read a paper on the relations of the efferent 

 branchial blood-vessels to the circidus cephalictis in the 

 Teleostean fishes, based upon an examination of specimens of 

 sixty-one species. He demonstrated the great variation that is 

 met with in the arrangement of the efferent vessels, and dis- 

 cussed the possibility of utilising the characters as a means of 

 arriving at a natural classification of the group. — Mr. G. A. 

 Boulenger, F.R.S., read a paper on the reptiles, batrachians 

 and fishes collected by the late Mr. John Whitehead in the in- 

 terior of Hainan. The collection contained specimens of fifteen 

 species, embracing four species of reptiles, six of batrachians, 

 and five of fishes. Of these, two species of reptiles, three of 

 batrachians, and three of fishes were described as new. — A com- 

 munication was read from Dr. A. G. Butler on a collection of 

 butterflies made by Mr. Richard Crawshay in British East 

 Africa. Sixty-eight species were enumerated and remarked 

 upon, of which four were described as new. — A second com- 

 munication from Dr. Butler contained a list of a small collectiorv 

 of butterflies made by Captain Hobart, of the Grenadier Guards, 

 in the Nandi District of the Uganda Protectorate. Of the 

 seventeen species represented in the collection, one (Cyw^^Mf?*? 

 hobarti) was described as new. — A communication was read 

 from Mr. J. Y. Johnson, containing a note on the habit and 

 mode of growth of the corals belonging to the genus Pleuro- 

 coralliuvi. — Mr. W. E. de Winton made some supplementary 

 remarks to those published in the Proceedings for 1898 (p. 900), 

 on the moulting of the King Penguin {Aptenodytes pennanti) now 

 livingtin the Society's Gardens. 



Entomological Society, November 15. — Mr. G. H. 

 Verrall, President, in the chair.— The President announced' 

 the death of Dr. C. G. Thomson, one of the honorary fellows 

 of the Society. — Mr. J. J. Walker exhibited four examples of a 

 species of Curculionidae- C/<;(?;»/.c sulcirostris, taken on red 

 sandy soil at Barr's Hill, near Oxford. These examples, he 

 pointed out, were of a reddish tint, harmonising with the colour 

 of the soil on which they were found, and in marked contrast 

 to that of normal grey specimens, some of which, taken at Deal 

 and Reading, he showed for comparison.— The President ex- 

 hibited specimens of Chersodromia hirta, which were found 

 under sea-weed at Brora in August 1899.— Mr G. W. Kirkaldy 

 exhibited two species of Hemiptera of economic interest, 

 one a Vyxxochond— Dysdercus cingtdatus (Fabr.), sent by Mr. 

 E. E. Green from Ceylon, where it was found appearing in 

 abundance on the cotton plants, the other a Psyllid— ^/^y- 

 rodicus dugesii Cockl. , forwarded by M. A Duges, who stated 

 that it is attacking the white mulberries in Mexico.— Mr. J. H. 

 Leech contributed part iii. of his paper on Lepidoptera Hetero- 

 cera from Northern China, Japan, and Corea. 



Cambridge. 

 Philosophical Society, November 13. — Mr. Larmor,. 

 President, in the chair —Intumescences on Hibiscus vitifolius, 

 by Miss E. Dale. Hibiscus vitifolius is a plant which is common 

 in the hotter parts of Asia, Africa and Australia. It is iisually 

 very hairy, but the hairiness is subject to considerable variation, 

 and the hairs are of several different kinds. On plants growa 

 in greenhouses there are formed abnormal outgrowths of the 

 epidermal and the subepidermal tissue of the stems and leaves. 

 These emergences, especially on the leaves, usually bear stomata 

 at their apices, and those on the stem are characterised by the 

 formation of a cork-cambium at the base of each, which cuts ofT 

 the older outgrowths. No traces of fungi or mites have been 

 found, but experiments show that these outgrowths are not 

 formed on plants in the open, where transpiration is more freely 

 promoted. Seedlings and cuttings from such plants devoid of 

 outgrowths, when placed in a greenhouse, develop outgrowths, 

 while seedlings from plants provided with them, and which 

 themselves have developed them, lose them if transferred to the 

 open. It seems therefore probable that growth under glass may 

 promote the production ot some actively osmotic body in the 



