ii8 



NATURE 



[May 31, 1900 



the ravages of the oHve fly. Hence the quantity of olive oil 

 obtained this season in Tuscany has been insignificant, while the 

 quality of most of it is distinctly inferior. A full crop of olive 

 oil may be reckoned at a money value of some 10,000,000/. 



The olive maggot — which subsequently develops into the 

 olive fly — destroys the pulp of the fruit, and so potent are the 

 ravages of this pest that it is capable of diminishing the yield of 

 oil by one-half, and seriously injuring the quality of the remainder. 

 It will therefore be seen that the fly may actually cause damage 

 in one year amounting to 5,000,000/. Notwithstanding the 

 urgency of the matter, no means of destroying the insect appear 

 so far to have been discovered, nor has the State suggested any 

 practical remedy. The subject is recommended to the notice 

 of English men of science, as any discovery which should ex- 

 terminate the plague ought certainly to be profitable. What 

 seems to be wanted is that entomologists of experience should 

 carefully study the habits of the fly with a view to finding out 

 the hitherto undiscovered winter habitat. Then alone could 

 proper steps be taken for its destruction. It has been hazarded 

 with some likelihood that the winter habitat of the fly must be in 

 the bark of the olive trees. If that were the case, all that would 

 be needed would be to paint the trees during the winter with a 

 simple solution of lime, which, though it might spoil the beauty 

 of the Italian landscape, would rid the country of a very formid- 

 able enemy to its agricultural prosperity. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — Two University lectureships in experimental 

 physics are now vacant. The appointment is for five years, 

 and the stipend 50/. a year. Applications should reach the 

 Vice-Chancellor by June 2. 



The researches of Mr. L. N. G. Filon, advanced student, of 

 King's College, in relation to certain problems in applied 

 -mechanics, have been approved as a qualification for the B.A. 

 degree. 



Sixty-three men and nineteen women have acquitted them- 

 selves so as to deserve honours in the Mathematical Tripos, 

 Part I. 



Honorary degrees will, on June 12, be conferred on the Earl 

 of Rosse, F.R.S., Sir Benjamin Baker, F.R.S., Sir W. L. 

 Buller, F.R.S., Prof. S. P. Langley, Prof. W. M. Flinders 

 Petrie and Prof. H. Poincare. 



The graces for the establishment of a new special examination 

 in agricultural science for the B.A. degree was opposed on 

 May 24, but it was carried by a large majority. The first 

 examination will take place at the end of the year. 



One of the chief difficulties which has to be overcome by 

 Technical Education Committees is the defective character of 

 elementary education, respecting which lament is very general. 

 Several instances of this difficulty are given in the current number 

 of the Record of Technical and Secondary Education. The 

 Durham committee have been compelled for some years to give 

 financial assistance to preparatory classes now formed in all but 

 twenty-one districts of the county. The committee have by 

 such means paved the way for their new regulation of 1899 that 

 there must be " the production of evidence of preparatory 

 training on the part of all new applicants on whom attendance 

 grants would be claimed." This action already appears to be 

 having a satisfactory effect. The Cambridgeshire, Nottingham- 

 shire and Staffordshire committees also deal at some length with 

 the question of defective elementary education. The Cambridge- 

 shire committee go so far as to say : — "The very backward state 

 of elementary education has made it extremely difficult, if not 

 impossible, to establish a system of technical education in the 

 proper sense of the term." The Staffordshire committee speak 

 of it in its relation to "the early age at which pupils leave the 

 elementary schools," and this has thrown upon theni "much 

 elementary and preparatory work which otherwise would have 

 been unnecessary." The importance of promoting the efficiency 

 of the work of evening continuation schools cannot be too 

 strongly urged, as they largely constitute the foundation of the 

 work of Technical Education Committees and thus lead on to 

 higher and specialised instruction. 



NO. 1596, VOL. 62] 



SOCIETIES AND ACADEMIES. 

 London. 

 Physical Society, May 25.— Prof. J. D. Everett, F.R.S., 

 Vice President, in the chair.— Prof. S. P. Thompson showed 

 some experiments illustrating the aberration called Coma. 

 If a converging lens is placed obliquely in a parallel beam of 

 light, instead of giving a point image, it produces unilateral 

 distortion, and the bright central spot is accompanied by a pear- 

 shaped tail, which is known as a coma. The direction in which 

 this tail points depends upon the side of the lens which is pre- 

 sented to the light. With a concavo-convex lens the convex 

 surface gives an inward pointing coma, and the concave surface 

 an outward pointing coma. The existence of this phenomenon 

 is due to unequal magnification from diff"erent zones of the lens, 

 a fact which was shown by covering the lens with a zone-plate 

 of three or four rings and viewing on a screen the distorted 

 images of the several zones. The form of a coma varies greatly 

 with the distance of the screen from the lens. A parallel beam 

 of light which has passed obliquely through a convex lens is 

 capable of producing some curious shadows. The shadow of a 

 rod can be obtained as a circular spot, and that of a grating, 

 made by stretching threads between two rods, as concentric 

 circular rings. Prof. Thompson also showed a stringed model 

 illustrating the paths of light-rays in the formation of a coma. 

 — Mr. R. T. Glazebrook then read some notes on the measure- 

 ment of some standard resistances. Three methods have been 

 employed by the author for building up multiples of a standard 

 resistance, such as a one-ohm coil. The first method consists in 

 making as accurately as possible three three-ohm coils. These 

 in parallel can be compared directly with the standard by 

 Carey Foster's method. Their resistance in series is very 

 approximately nine times that in parallel, and hence an accurate 

 determination of a resistance about nine ohms can be ob- 

 tained. If, then, this resistance is put in series with- the 

 standard, an accurately-known ten-ohm resistance is ob- 

 tained. By a similar process, a hundred- or a thousand- 

 ohm coil can be built up. The second method consists in 

 calibrating a resistance-box. The one-ohm coils of the box 

 are compared directly with the standard, and the other 

 resistances determined accurately by a building-up process, 

 using a subsidiary resistance-box. In comparing the high 

 resistances, the diff'erence between the two boxes may be so 

 great as to send the balance off the bridge wire. In these 

 cases the third method is employed. The equal arms of the 

 bridge are accurately known, and one of them is shunted with a 

 resistance, which need not be accurately known, until the read- 

 ing is brought back on to the wire. The coils chiefly used 

 throughout the experiments are made of platinum-silver. 

 — Mr. J. J. Guest read a paper on the strength of ductile 

 materials under combined stresses. The author through- 

 out his experiments has used the "yield point" of a material 

 as the true criterion of its strength, and has rejected the elastic 

 limit as being modified by local yielding. At present, two 

 theories are used in the calculation of strengths of materials. 

 The first is that the material yields when one of the principal 

 stresses reaches a certain amount. This theory, which was 

 adopted by Rankine and is used by engineers in England and 

 America, is not in accord with recent experiments. The second 

 theory is that the material yields when the greatest strain reaches 

 a certain amount. This was advocated by St. Venant, and is 

 used by engineers on the Continent. Besides these there is a 

 third theory of elastic strength, in which the condition of yield- 

 ing is the existence of a shearing stress of a specific amount. In the 

 case of a solid bar subjected to torsion, there is a variation in the 

 strain from the axis outwards, and consequently the materials have 

 been used in the form of thin tubes. This allows the application of 

 an internal fluid pressure. The specimens were of steel, 

 copper and brass, the state of set caused by drawing having 

 been removed by annealing. The tubes were subjected to (1) 

 torque, (2) torque and tension, (3) tension only, (4) tension 

 and internal pressure, (5) torsion and internal pressure, and 

 (6) internal pressure only. The axial elongation, the twist, 

 and occasionally the circumferential strain were measured. 

 Towards the end of the experiments observations were made 

 on bending. The results disprove the maximum stress 

 theory, and are at variance with the maximum strain theory. 

 The maximum shearing stress developed, and the correspond- 

 ing shearing strain were comparatively constant through- 

 out the experiments, and no other simple relation between the 

 stresses or strains was even approximately constant. The results 



