^98 



NdTjUJiE 



[May 5, 192 1 



50 c.c. Burettes (with tap). N.P.L. limits : — Maximum error allowed at any point, and also maximum difference allowed between 

 the errors at any two points :— Class A, ±0'04 c.c. Class B, ±0*07 c.c. The delivery times specified depend on the length occupied 

 >v by the graduations. For lengths between 50 cm. and 55 cm. the times are : — Class A, iro-220 sec. ; Class B, 75-220 sec. 



The results marked *, i.e. those which relate to 

 apparatus bearing well-known British trade-marks, 

 are very satisfactory. Of the twenty pieces of 

 apparatus fhus marked only five have capacity errors 

 in excess of the Cldss B limits, and of these four 

 have errors but slightly in excess of the Class B 

 limits, the only really unsatisfactory piece of apparatus 

 being the 10 c.c. pipette in error by +0065 c.c. 



The results for the German apparatus clearly show 

 that such apparatus cannot ^be accepted on trust, as 

 many users appear to imagine. For example, the 

 50 c.c. German pipettes include a number which have 

 excessive errors and delivery times so short as to 

 render them likely to give inconsistent results. The 

 ordihary grade German apparatus has no claim to 

 superior accuracy as compared with ordinary- grade 

 British apparatus. 



The results obtained indicate that where accuracy 

 is of importance apparatus should be tested before 

 use, atjd that in cases where untested apparatus is to 

 be used the safest procedure is to obtain apparatus 

 bearing the trade-mark of British firms which make a 

 feature of siipplying apparatus marked by the National 

 Physical Laboratory. Their experience in the manu- 

 facture of such apparatus is clearly reflected in the in- 

 creased accuracy of their ordinary output as compared 

 with apparatus not bearing their trade-marks. 



J. E. Petavel, 



Director. 



The National Physical Laboratory, April 16. 



Young's Interference Experiment. 



In connection with Dr. Houstoun's letter on this 

 subject in Nature of April 28, I may direct attention 

 to a note by my father " On a Simple Interference 

 Arrangement" (British Association Report, pp. 703-4, 

 1893; Collected Works, vol. iv., p. 76). . The 

 arrangement described is a tube with a single slit at 

 one end and a double slit at the other. The double 

 slit is ruled on silvered glass, and is much closer 

 than Dr. Houstoun describes — about i/io mm., as 

 nearly as I can estimate without pulling the apparatus 

 to pieces. The slit at the other end is about 035 mm., 

 and the length of the tube about 31 cm. The ap- 

 paratus as originally described was intended to be 

 used with the double slit placed close to the eye and 

 the tube directed to a source of light such as a 

 gas flame or the sky. Interference bands are then 

 well seen. ; 



About two years before my father's death I remem- 

 ber asking him whether he had ever seen Young's 

 NO. 2688, VOL. 107] 



experiment tried. He replied in the negative, and 

 suggested trying it. It was during a vacation, and I 

 had leisure to do so. In looking round for the neces- 

 sary appliances it occurred to me that the " simple 

 interference arrangement " was just, what was 

 wanted. I placed it in a hole in the shutter of a 

 dark room, with a heliostat outside, so that the single 

 slit was backed by the reflected image of the sun. 

 No lenses were used. The interference bands were 

 then admirably seen on a card held, so far as I 

 remember, 2 ft. or 3 ft. away from the double slit. 

 My father remarked that he did not remember to 

 have seen interference bands projected so well before 

 with any arrangement. 



It will be noticed that this use of the apparatus is 

 not materially different in principle from the original 

 subjective use. To pass from the one arrangement to 

 the other we have only to substitute the card screen 

 for the retina and to increase the focal length of 

 the lens of the eye without limit, at the same time 

 removing the screen to a great distance. 



Rayleigh. 



Terling Place, Witham, Essex, April 29. 



An Addition to the British Fauna 



{Rhynchodemus Scharffi). 



One of my students, Mr. G. D. Morison, has brought 

 me a very interesting Planarian worm which he 

 found this morning under rotten wood in a coal-shed 

 in a garden at Chiswick. This is the fourth speci- 

 men which he has found in the same place, where 

 they were associated with slugs, upon which they 

 may have been feeding. The other three specimens 

 were found a week ago. A living specimen was 

 measured up to 6 cm. in length when crawling. In 

 this condition the worm is very slender ; when con- 

 tracted it is naturally a good deal thicker and exhibits 

 a transverse wrinkling. The colour in life is usually 

 bright yellow, without pattern, but with a dusky 

 anterior tip bearing the two small eyes close to the 

 extremity. Mr. Morison tells me, however, that one 

 specimen was salmon-pink. 



The specimen submitted to me (alive) agrees very 

 closely with von Graff's description and figures of his 

 Rhynchodemus Scharffi, and I have- no hesitation in 

 identifying it with that species, which was first dis- 

 covered in 1894 in a Dublin garden, and has not, so 

 far as I am aware, been recorded since. Von Graff 

 at first supposed that the species had been introduced 

 from the tropics, but afterwards adopted Dr. Scharff's 

 view that it js indigenous to Ireland. It Is hard to 



