772 



NA TURE 



[December g, 1922 



speed varies as the thickness of the turning. As the 

 turning does not begin to curl or bend until after it 

 passes the edge, it is not .difficult to see why the edge 

 should wear well and give the impression that it 

 had not been in contact. The fact that the point 

 of the tool is in contact with the work may be inferred 

 from the fact that in many circumstances steel is 

 welded on to the point, and but for the greater 

 relative motion behind the cutting edge where the 

 scouring action is excessive, this deposition of metal 

 would probably be more extensive. 



We are then driven back on another part of the 

 problem. Why do shavings curl ? The analogy with 

 rivet heads is unconvincing, for shavings are universally 

 flat in a lateral direction, winch — having regard to 

 variety of tool profiles — is evidence of the extra- 

 ordinary stresses involved and of the flow they 

 produce — the stress on the upper face of the tool 

 is of the order of ioo tons per square inch in quite 

 ordinary practice. A more direct and convincing 

 explanation is the following. Consider a piece of the 

 shaving as in the diagram (Fig. i). There is the down- 

 wards shearing force S at the principal plane of shear 

 and an opposite reaction R at the tool face. These 

 produce a turning couple which has more than one 

 effect. In most cases the effect of this couple is 



to distort the emerging element into a wedge the 

 base of which is extended by plastic flow at the tool 

 face and the upper part is shortened as in bending. 

 The tendency is to place the lower or bearing surface 

 of the shaving in tension as in a beam, and if the 

 material has a low tensile strength as in cast iron, 

 the shavings break, but even these brittle materials 

 show a slight curl in the small particles removed by 

 the tool. Friction at the tool face, as Mr. Mallock 

 points out, resists curling, but it is probably not the 

 governing factor. In the discussion on Prof. Coker's 

 paper, Mr. H. I. Brackenbury put forward the very 

 important observation that slow speeds are conducive 

 to curling and high speeds produce straight shavings 

 — this in tough steel. Having regard to the fact 

 that cutting-tool action is a problem in plastic flow, 

 the viscosity of the metal is probably important ; 

 and as the temperatures at the moment of cutting 

 are very high, conductivity and tensile strength when 

 hot may be decisive factors. The ratio of shearing 

 strength to tensile strength enters into the phenomenon 

 of curling, but as parts of the shaving are flowing 

 it is not easy to get clear views on what is taking 

 place. H. S. Rowell, 



Director of Research. 

 Research Association of British Motor 

 and Allied Manufacturers. 

 15 Bolton Road, Chiswick, W.4. 

 November 9. 



An Empire Patent. 



Mr. Hulme's letter in Nature of November n, p. 

 633, raises objections to the Empire Patent on grounds 

 which would occupy too much space to traverse fully, 

 but I would venture to question his general con- 



no. 2771, VOL. r 10] 



demnation of the present patent system. A system 

 such as he proposes, which would grant a monopoly 

 only to such persons as were actually working an 

 invention, would be unjust to an inventor without 

 capital to exploit his ideas. Moreover, considerations 

 of novelty could not entirely be left out. Presumably 

 Mr. Hulme would leave this to be decided by the 

 Courts ; but litigation is costly, and I imagine that 

 few concerned with the business side of patents 

 would be willing to dispense with a search for novelty 

 on the part of the Patent Office, the advantages of 

 which appear to be sufficiently obvious. The limita- 

 tion, for practical reasons, to British patent specifica- 

 tions does not detract from those advantages, for, 

 assuming the patent system to be of some value, it 

 is most likely that subject-matter of a patentable 

 nature will be disclosed in the first place in a patent 

 specification. Mr. Hulme's objections to the search 

 appear to be based largely upon United States ex- 

 perience, but the opinion he quotes may be due to 

 difference of efficiency in administration in the two 

 countries, particularly when we consider that Ameri- 

 can search is theoretically not so limited as here. 



The arrangements as to " working " laid down 

 in the Patent Act of 19TO, which ensure that any 

 patent granted in this country must be worked here 

 on a commercial scale, would, it is to be presumed, 

 apply to an Empire Patent. 



In conclusion, may I be allowed a word of warning 

 on the too facile application of biological considera- 

 tions to human society ? Mr. Hulme's assumption 

 that a flattening of the population curve is an un- 

 favourable symptom would not be endorsed by 

 sociologists, and tends to discount whatever force 

 there may be in his biological deductions re Patent 

 Laws. Ernest E. Towler. 



35 St. Andrews Square, 



Surbiton, November 15. 



The Movement of the Positive After-image. 



There is no doubt that movement of the positive 

 after-image takes place without movement of the 

 eyes as Mr. H. S. Ryland states (Nature, November 1 8, 

 p. 668). His experiment is complicated by the fact 

 that all portions of the light shown have not the same 

 intensity, causing a corresponding difference in the 

 after-image. 



There appears to be, at any rate while the eyes 

 are being used, a steady circulation of photo-chemical 

 material from the periphery to the centre of the retina. 

 The following experiment shows very simply the 

 movement of the positive after-image without moving 

 the eyes. If two rectangular strips of white paper 

 about three inches long and a third of an inch wide 

 be placed on a piece of black velvet and separated by 

 a distance of an inch, definite positive after-images 

 may be obtained of the two strips by viewing them 

 with one eye, the eye being directed to a point mid- 

 way between the two strips of paper, the other being 

 closed and covered with black velvet, for the shortest 

 possible time, the eve being simply opened and closed. 

 Two clear-cut positive after-images will first be seen ; 

 these rapidly become blurred and gradually approach 

 each other, the central portions of each appearing to 

 bulge towards each other and to combine first ; the 

 upper and lower portions disappear first, the two 

 after-images gradually combine in the centre of the 

 field of vision, the last phase being a white circular 

 blur, which slowly disappears with a whirlpool move- 

 ment. It will be noticed that the after-images do 

 not become negative. F. \Y. Edridge-Green. 



London, November 18. 



