April 24, 19 19] 



NATURE 



155 



OPHTHALMOLOGICAL TRAINING OF \ 

 MEDICAL STUDENTS. I 



"T^ HE Council of British Ophthalmologists has 

 ■*■ issued a report dealing with the teaching and 

 examination of medical students in ophthalmology. 

 The first part of the report reviews briefly the efforts 

 made up to the present by the General Medical 

 Council to ensure better training of the medical 

 student in this important subject. These, unfor- 

 tunately, have not succeeded in their object, and it 

 is still the case that "the general body of the medical 

 profession does not possess a competent knowledge 

 of diseases of the eye." 



The second part of the report deals in detail with 

 the requirements of all the examining bodies in Great 

 Britain and Ireland and, for comparative purposes, 

 with a large number of Colonial, American, and 

 foreign universities. The analysis of these require- 

 ments shows that Great Britain stands almost alone 

 in_ granting diplomas to practise medicine without 

 evidence of an adequate knowledge of diseases of the 

 eye. In Ireland and irf the great majority of foreign 

 and Colonial universities ophthalmology is one of the 

 subjects of the qualifying examination, and the 

 examinations in it are conducted by ophthalmic 

 surgeons. 



The council has therefore recommended (i) that no 

 student shall be admitted to the final examination, 

 qualifying to practise medicine, unless he has attended 

 an ophthalmic clinic for not less than six hours a 

 week during a period of three months, and has 

 attended a course of systematic instruction in 

 ophthalmology; and (2) that no student shall be con- 

 sidered to have passed the qualifying examination 

 unless he has shown a sound knowledge of practical 

 ophthalmology in an examination conducted bv 

 ophthalmic surgeons. 



T 



CLOCK ESCAPEMENTS.^ 

 HE most ancient instruments for measuring time 

 were probably some kind of sundial. Some^ 

 thing of the kind is, no doubt, referred to in 

 2 Kmgs XX. and Isaiah xxxviii., where it is stated 

 that the shadow moved back ten steps on the steps 

 of Ahaz (for that is the literal translation). Hero- 

 dotus (" Euterpe," cix.) tells us that the Babylonians 

 introduced to the Greeks the jtoXos and the yvdifMou, 

 no doubt some forms of sun-instruments. Frequent 

 allusions are found in the classics to the clepsydra, 

 which was made in various forms, always depending, 

 however, upon the approximately uniform flow of 

 water through a small hole. 



But clocks, properly so called, cannot be traced 

 with certainty earlier than the fourteenth century. 

 In 1348 a curious iron clock was sent over from 

 Switzerland, and was until recently kept in Dover 

 Castle. It is now in the Science Museum at South 

 Kensington. It is interesting as having no pendulum 

 or balance-spring (both much later inventions), but, 

 instead, a vertical spindle carrying a horizontal 

 traverse loaded at the ends with weights. This 

 vertical spindle has two pallets projecting from its 

 sides, approximately at right angles to each other, 

 which engage alternately the uppermost and lower- 

 most tooth of a contrate wheel the axis of which is 

 horizontal and in the same plane with the vertical 

 axis first referred to. This is the " verge " escapement, 

 which was for long afterwards used in both clocks 

 and watches. No good timekeeping was possible 

 with such an arrangement. Gravity did not come 



1 From a discourse delivereil at the Roy.il Institution on February at by 

 A. T. Hare. 



NO. 2582, VOL. 103] 



into the problem, and the speed of the movement 

 was only restrained by its energy having alternately 

 to create and destroy angular momentum in the 

 swinging arms. The force of the train, however 

 variable, was paramount. 



The next step in horology, and undoubtedly the 

 most important which has ever been made, was the 

 application of the pendulum to clocks by the Dutch 

 physicist and astronomer, Christian Huygens, in 

 1657. Galileo had discovered, about sixty years 

 earlier, the isochronism (since found to be only ap- 

 proximate) of a swinging body, but, in spite of efforts 

 made after his death to claim priority for him in the 

 invention of the pendulum clock, the evidence has 

 not convinced historians of his title to that honour. 



Huygens, being aware of the fact that the motion 

 of a particle under gravity was only isochronous, 

 independently of the extent of the arc of swing, when 

 the body describes a cycloid, and knowing the pro- 

 perty of that curve to reproduce itself as an involute 

 of an equal cycloid, attempted to secure the desired 

 isochronism by suspending his pendulum from a silk 

 thread which swung l>etween two cheeks of brass cut 

 to the shape of the cycloid, thus obliging the bob to 

 trace an involute. But the silk was so affected by 

 the weather that no good result ensued. 



Another objection to the verge escapement was the 

 large arc of swing necessary to permit the escape- 

 ment to unlock itself. Huvgens attempted to over- 

 come this difficulty by making the verge the axis, 

 not of the pendulum-crutch, but of a pinion gearing 

 into a larger wheel to the arbor of which the crutch 

 was attached. This construction permitted the angle 

 of swing to be reduced at pleasure, but more friction 

 was introduced, and little improvement was effected. 



The calculation of the time of swing of a free 

 pendulum describing a circular arc can only be made 

 approximately, but the approximation can be carried 

 as far as desired, and as the arc of swing is never 

 large, a few terms suffice. This is the formula : — 



T= — ,-1+ sm2 -4-7 srn* + ... 



from which, by differentiation, 



{iT_'irk^na/ o • •.« \ 



(fa 16 Jj^aK, 2 • • -J- 



Here T is the time of swing of the pendulum from 

 its highest position to the vertical, and o is the semi- 

 angle — that is, the angle turned through from the 

 highest to the lowest position. Now of the factors 

 making up the expressions on the right-hand side 

 of these equations, only it and g and the numerical 

 coefficients can really be considered as constant. It 

 has been suggested that even g may one day be shown 

 to be variable. As for h and k — that is, the distance 

 from the axis of motion to the centre of gravity and 

 the radius of gyration respectively — these are well 

 known to be dependent on temperature, and an 

 interesting account might be given, if time permitted, 

 of the evolution of the compensated pendulum. The 

 recent discovery of alloys of iron and nickel the 

 coefficient of expansion of which is very low has 

 much facilitated this. 



The factor which has most influence on the value 

 of T is o, the angle of swing. The formulae show 

 us two things : first, that the wider the arc of swing 

 the more a clock will lose, and, secondly, that a given 

 small variation of arc is less harmful when the w;hole 

 arc is small than when it is great. There are prac- 

 tical reasons, however, for not making it too small, 

 which have led to the adoption of arcs of two or 

 three degrees on each side of the vertical as, on the 

 whole, the best. 



