1 88 



NATURE 



[June 



2 1, 1900 



oscillatory, and, therefore, account must be taken of the pressures 

 involved in producing changes of motion, in consequence of 

 which the pressures differ from those which would exist at any 

 instant if the motion were steady. These pressures, which I 

 have called the " d<^ldt pressures," after the term which pro- 

 duces them in the hydrodynamical equation, are completely 

 modified in their action when continuous motion is replaced by 

 discontinuous motion, but their efifect can only be discussed 

 from general principles. Other points are the effect of the 

 ship's forward motion in increasing the steadying action, and the 

 influence of bilge-keels in modifying pitching and in improving 

 the steering of ships ; the two last effects are further simple 

 consequences of the properties of discontinuous motion. 



We have seen that the action of bilge-keels in steadying a ship 

 is largely modified by the actions of the sides of the ship, and is 

 much augmented when the keels are placed in a suitable position ; 

 and it is interesting to notice how the exigencies of trade, while 

 they have necessitated the removal of keels from the middle 

 line of the ship to the sides, have brought about such a change in 

 the form of the section as to render the new position by far the 

 most effective. In the old pointed-bottomed ship, the central 

 keel was the best, as it not only intercepted the currents flowing 

 round the bottom in each swing from side to side, but also 

 produced a difference of pressure on the two sides whose moment 

 was always opposed to rolling. In the modern flat-bottomed 

 ship of rectangular section a central keel would be unfavourably 

 placed for this purpose, as not only would the water tend 

 to flow in the same direction as that in which the ship is rolling, 

 producing a diminished pressure on the keel, but the differences 

 of pressure on the sides of the ships would have a moment 

 tending to diminish the resistance to rolling. The favourable 

 position now commonly assigned to the bilge-keel is calculated 

 to render its addition to ships of the greatest value in increasing 

 their steadiness. G. II. Bryan. 



THE ''ORTHOSTIGMAT" LENS. 



A SPECIMEN of the new series of lenses issued under 

 "^^ the joint auspices of Messrs. Beck and Steinheil, has 

 been tested and examined by us. The lens is of the rectilinear 

 symmetrical type with two equal combinations, each consist- 

 ing of three elements. It is by special construction of the 

 surfaces of these components that the remarkable qualities 

 claimed for, and undoubtedly possessed by, the new lens are 

 attained. The great drawback to the best of the old type lenses 

 was the curvature of field, and it is only in recent years that 

 the discovery of the new varieties of glass has made it possible 

 to correct this, and at the same time perfect the corrections for 

 spherical aberration and astigmatism. The result of the pro- 

 cess is that each component of the lens is made up of a positive 

 meniscus, with a double convex lens cemented on one side of 

 it and a double concave one on the other. The centre positive 

 lens will consequently have a lower power than the two outer 

 ones. 



When it is understood that each of the twelve surfaces in- 

 volved in the complete lens is worked with such accuracy that 

 an error of i/40,oooth of an inch is inadmissible, the increased 

 price, compared with the old types of lenses, is amply accounted 

 for in the superior product obtained. 



We have tested the lens, which is of about 4f inches solar 

 focus, and are quite satisfied that ic fulfils to a high degree of 

 perfection the claims made for it by the makers. Although 

 the lens is listed for \ plate, the circle of good definition 

 is much larger, and with a stop of //16 excellent definition 

 was obtained over the whole of a | plate. The lens at its 

 greatest rapidity works at //6'3, and at this aperture the 

 definition appears very perfect over an area 4^ inches 

 square. The makers call attention to the special attempts 

 they have made to eliminate astigmatism, and it is not until an 

 oblique pencil falls considerably beyond the listed angle that 

 any distortion shows itself. To make this clearer, let the 

 image of the sun, moon, a star, or any distant object, be 

 focussed at the centre of the plate, and then the camera so 

 tilted that the image falls gradually away to the extreme corner. 

 The slightest distortion can be at once recognised in this 

 manner. With the lens in question no distortion was evident 

 until the image was received at an angle of about 30 degrees 

 from the axis, and for another 10 degrees further the resulting 

 astigmatism, although present, was much less than is present 



NO 1599. VOL. 62] 



closer to the axis in a rectilinear of the ordinary type. So that 

 for an angle of n iew of over 60° the new lens is practically non- 

 astigmatic at the large aperture //6'3. This will recommend 

 it especially for all process and copying work, where critical 

 definition and speed are primary necessities. A word should 

 be said concerning the focussing with these new lenses. This 

 adjustment must be critical, as an almost imperceptible dis- 

 placement of the plate will suffice to throw it out of the focal 

 plane sufficiently to destroy the definition, and so create false 

 impressions of the capabilities of the optical system. For all 

 such work, therefore, only perfectly rigid apparatus is per- 

 missible. 



Another important qualification of the lens is its comparative 

 freedom from chromatic aberration, in virtue of which it will 

 be useful for work connected with colour photography, obviat- 

 ing the laborious and uncertain corrections which are necessary 

 in such work with the ordinary lenses, whose chromatic aber- 

 rations are only suppressed for the blue and yellow. Critical 

 tests of this correction have not been possible, but sufficient 

 have been made to show that the outstanding error is small. 



On account of their covering power, the smaller sizes will be 

 found excellent for low-power microphotography. The new 

 lenses are obtainable with foci varying from 34 in. to 23^ in., 

 covering plates from 3^ in, square to 28 x 24 inches. 



UNIVERSITY AND EDUCATIONAL 

 LNTELUGENCE. 



Oxford. — The Romanes Lecture will be delivered by Dr. 

 James A. H. Murray on Friday, June 22, upon " The Evolution 

 of English Lexicography." 



Sir William Thiselton-Dyer, F.R.S., has been made a per- 

 manent elector to the Sherardian Professorship of Botany, in 

 succession to the late Prof. Bartholomew Price. 



The Statute instituting Diplomas in Geography has been 

 approved by Convocation, and is to remain in force until 

 October, 1904. 



The extensive bequest to the University under Mr. Fortnum's 

 will has made an enlargement of the Ashmolean Museum 

 necessary. The cost is estimated at 1500/., and towards this 

 sum Brasenose College has offered a contrii)Uiion of 500/. 



At Merton College there will be an election to a fellowship 

 on October 6, after an examination in Animal Physiology and 

 Animal Morphology. Candidates are requested to inform the 

 Warden by September 10 of their choice between these two 

 subjects, and to submit, if they wish, original papers or memoirs. 



At a meeting of the Oxford University Junior Scientific Club, 

 held on Wednesday, June 6, Sir John Burdon Sanderson, 

 F.R.S., gave an account of the method he has been lately em- 

 ploying for producing tetanus in muscle, by means of telephone 

 currents produced by musical sounds, showing how the results 

 bear on the vexed question of the inherent rhythmicality of 

 muscle and nerve-cell. The lecture was illustrated by experi- 

 ments. 



At a meeting of the above club held on Friday, June 15, Mr. 

 T. C. Porter, of Eton College, described the growth of the 

 shadow of the Peak of Tenerifi'e, as witnessed from the summit 

 of the mountain at sunrise and sunset, and its gradual eclipse by 

 the shadow of the earth. He showed photographs taken at the 

 time, and explained how by means of an ordinary watch and 

 pair of opera glasses a rough value of the diameter of the earth 

 might be deduced. 



Mr. A. F. Walden, New College, made a preliminary com- 

 munication to the club on the theory of labile hydrogen atoms. 



Cambridge.— The researches of Mr. J. C. McLennan on 

 electrical conductivity in gases traversed by kathode rays, and 

 of Mr. R. L. Wills, of St. John's College, on the magnetic 

 properties of iron as influenced by temperature and the 

 presence of other elements, have been approved by the Degree 

 Committee as qualifying for the B.A. degree. 



In the Natural Sciences Tripos, which is now the largest o. 

 the Honour Examinations, forty men and three women are 

 placed in the first class of Part I. In Part II. fourteen men and 

 no women obtain first-class honours. 



At St. John's College the following awards in natural science 

 were made on June' 18. Foundation Scholars continued or 

 elected : Lewton-Brain, May, Adams, Ticehurst, Fletcher, 

 Browning, Wakely, Gregory, Williams, Harding, Ilepworth, 



