March 29, 191 7] 



NATURE 



97 



varies the distance between the images as it slides 

 along", and this variable separation of the coUimated 

 images provides a convenient means for angular 

 measurement. If the scale is graduated to correspond 

 with angular separation, it can be used as a standard 

 of angular measurement for such purposes as mark- 

 ing olf or checking stadia intervals on levels or theo- 

 dolites, or for checking the graticules in prism bino- 

 culars. — P. F. Everitt • The design and testing of 

 telescope objectives. The author first described the 

 lour principal aberrations of the telescope objective, 

 proceeding from them to others of less importance, 

 and finally reducing the problem to the fulfilment of 

 three or more of six conditions which it is desirable 

 satisfy. After referring to the existing tables of 

 jroximate solutions, trigonometrical formulae are 

 ^iven by means of which selected rays are accurately 

 traced through an objective, and the aberration is 

 shown numerically in an example in which the 

 chromatic aberration of an objective is altered at will. 

 A short description of the main tj'pes of objectives 

 was given, showing the purposes for which they are 

 best adapted, and also some methods of testing, in- 

 cluding the Hartmann sjstem. 



Physical Societ}-, March 9.^ — Prof. C. V. Boys, presi- 

 ;. at, in the chair. — Dr. P. E. Shaw: To measure the 

 pressure in a high vacuum by observations of logarith- 

 ' lie decrement. In experiments on the Newtonian con- 

 nt (P/21Z. Trans., May, 1916^ the author used a torsion 

 lance in a vacuum which varied in different cases 

 from 15 mm. to o-ooooi mm. pressure. Before sealing 

 the vessel the pressure was determined by a McLeod 

 gauge. Values of the pressure after sealing off were 

 deduced, in the case of the higher vacua, from observa- 

 tions of the damping of the torsion system. The 

 formula employed is due to the late Prof. Poynting, 

 and can be expressed in the form 



P = 356 J-r^, 



where I = moment of inertia of suspended system, 

 s^area of surface (supposed plane) which is experi- 

 encing the resistance, a = mean distance of plane from 

 centre of rotation, T = period of oscillation, and A = the 

 observed logarithmic decrement. A table and curve 

 are given showing the relation between P and A. — 

 A. W. Clayden : A diffraction colour box. The appa- 

 ratus consists essentially of a ver}- simple concave 

 grating spectroscope, of which the slit and grating are 

 situated at opposite diameters of a circle, the spectrum 

 being formed on the arc of this circle. Two inde- 

 pendent arms carr}' fittings on which may be placed 

 either telescope eyepieces or small electric lamps. With 

 the slit of the instrument illuminated by a suitable 

 source, the eyepieces can be set so that any two desired 

 wave-lengths are in the centres of their respective 

 fields of view. The eyepieces are then replaced by the 

 small lamps (the filaments coinciding with the previous 

 positions of the cross-lines), and the grating is observed 

 with a small telescooe pointed towards the widened 

 slit ; the whole of its surface is seen to be illuminated 

 with a mixture of the two colours on which the eye- 

 pieces were originally set. The "concave grating" 

 employed consists of a Thorpe replica of a Rowland 

 plane grating of 14,475 lines to the inch, mounted with 

 its ruled surface in contact with the surface of a con- 

 cave mirror of 4-ft. focal length. This forms an ad- 

 mirable substitute for the more expensive concave grat- 

 ing. The author prefers to state results in terms of 

 the number of oscillations per unit of time. Obser\'a- 

 tions showed that the smallest change of wave-length 

 which could be recognised by the eye as a change of 



NO. 2474, VOL. 99] 



colour was greater than that which corresponded to 

 a change of period of 10'* vibrations per second, or 

 to a change of one vibration more or less in i/io'' 

 second. — Prof. W. M. Coleman : An apparatus for 

 studying the effect of Hertzian waves on the heart. A 

 simple pendulum, consisting of a c\iindrical brass 

 bob terminating in a pointed wire coaxial with the 

 bob, hangs by a piece of string above one of the ter- 

 minals of an induction coil, so that in its lowest posi- 

 tion the point of the oob is within sparking distance 

 of the terminal and vertically above it. The bob is 

 'connected by a piece of flexible wire to the other ter- 

 minal of the coil. When the pendulum is set oscil- 

 lating there is a shower of sparks every time the bob 

 passes its lowest position. The frequency of inter- 

 mittence can be varied by altering the length of the 

 suspension. By adjusting the period of the pendulum 

 nearly to the time of a heart-beat any possible effect 

 on the rate of the beating may be observed. The 

 condensed discharge from two Leyden jars is employed. 



Geological Society, March 14.— Dr. A. Smith Wood- 

 ward, vice-president, in the chair. — L. M. Parsons : 

 The Carboniferous limestone bordering the Leicester- 

 shire coalfield. The inliers of Carboniferous limestone 

 situated along the northern border of the Leicestershire 

 coalfield crop out in two well-defined series : a w^estern 

 series composed of almost horizontal beds exposed by 

 stream-erosion, and an eastern series in which the 

 limestone is highly inclined and complicated by fault- 

 ing. The thinly bedded limestones, shales, and dolo- 

 mites of the western inliers are of a slightly higher 

 horizon than that of the uppermost beds of the more 

 massive dolomites seen at Breedon and Breedon 

 Cloud farther eastwards. In no part of the district 

 is the base of the Carboniferous seen, although borings 

 have shown that the limestone rests upon pre- 

 Cambrian rocks in the neighbourhood of Chamwood 

 Forest. 



Linnean Society, March 15. — Sir David Prain, presi- 

 dent, in the chair.— C. E. Jones : Methods of preparing 

 plants for exhibition. The experiments described 

 have been carried out in connection with the exhibi- 

 tion of plants in the Department of Botanj-, Natural 

 History Museum, South Kensington, where sf>ecimens 

 of the results can be seen (see also Nature. Novem- 

 ber 9, 1916, p. 191). — Dr. R. R. Gates : A systematic 

 studv of the North American Melanthaceae from the 

 genetic standpoint. The author's point of view is the 

 assumption, based upon experiment during the last 

 fifteen years, that the variations which mark species 

 have not been universally continuous and infinitesimal, 

 but often definite and discontinuous. Definite varia- 

 tion is not necessarily orthogenetic variation, but 

 marked variation which may occur in any, or in many, 

 directions simultaneously. The experience gained in 

 work on the mutations in Oenothera are turned to 

 account in this group of Liliales which has not hitherto 

 been the subject of experiment. Pairs of species have 

 been taken and investigated on this basis. Related 

 genera showing marked differences in structure often 

 co-exist side by side, showing that these differences 

 cannot be claimed as of selective value, but have 

 arisen from " spontaneous variation " and have been 

 perpetuated by hereditA,-. 



Mineralogical Society, March 20. — -Mr. W. Barlow, 

 president, in the chair. — A. Holmes and Dr. H. F. 

 Harwood : The basaltic rocks of Spitsbergen and 

 Franz- Joseph Land, with conclusions regarding the 

 Brito-.\rctic Tertiary* Petrographic Province. These 

 rocks, which were obtained respectively from Prof. 

 Garwood and the Geological Sur\-ey of England and 

 Wales, are very similar not only to the basaltic rocks 

 previoush- described from neighbouring localities, but 



