September i, 192 i] 



NATURE 



I r 



A / >. 3 



Using an unsoftened needle so mounted that verj' 

 light pressure might exist between it and the grind- 

 ing stone, a point was produced which appeared per- 

 fectly sharp under a microscopic power of 300 

 (Fig. 4). How sharp this really is is quite uncertain, 

 for probably the same appearance would be presented 

 whether the radius of curvature were one-half or one- 

 hundredth of a wave-length of the light used in its 

 examination. 



I believe, however, that, short of molecular dimen- 

 sions, there is no limit to the sharpness attainable 

 if sufficiently light pressures are used in grinding. 



In order to get some idea of the chance which 



a fine steel point would have of surviving when in 



use, I dropped a needle weighing about 3 grains 



through a distance of i in. on a glass surface. 



The result is shown in 



Fig. 5- 



It would seem, there- 

 fore, that though it might 

 be just possible to rule 

 lines separated by 

 1/100,000 in. with a steel 

 point, great care would be 

 required, especially in 

 bringing the point into con- 

 tact with the film at the 

 beginning of each line. 



What happens in the 

 process of grinding is in- 

 dicated in Fig. II, a, b, c. 

 Suppose that a truncated 

 cone (a) is being ground, 

 and that the pressure be- 

 tween it and the stone is 

 at first uniformly distri- 

 buted along the line AB ; 

 j as grinding proceeds it is 

 evident that the pressure 

 I near the small end will 

 g i diminish, owing to the 

 necessaiy bending and 

 shear set up at that part. 

 Continued grinding either 

 i causes the tip to break off 

 (b), or forms a quasi-cylin- 

 drical end (c), according to 

 the brittleness or pliability 

 of the material. (The 

 wire - like end (c) is 

 analogous to the " wire edge " formed when grinding 

 a slightly soft knife or chisel.) 



The harder the material and the more obtuse the 

 cone, the nearer to the geometrical apex will the 

 break occur. 



In the ground diamond points which I have 

 examined the end is rounded, and the lines of diffrac- 

 tion gratings ruled by them are merely ver\- shallow 

 grooves, as may be seen if thin sections are made 

 of celluloid or gelatine casts. Casts taken from a 

 Rowland grating of about 17,000 lines per inch gave 

 spectra nearly as brilliant as the grating itself, but the 

 depth of the grooves was not distinguishable even 

 when viewed with an immersion twelfth objective. I 

 have been told that in ruling these gratings the load 

 on the point was 3 or 4 grains, and if this is 

 the case the whole of the ruled surface must have 

 been sunk below that of the original level. In casts 

 from some of my own gratings of 5000 per inch, the 

 grooves, though shallow, could be readily distin- 

 guished. Each groove was the full 1/5000 in. across, 

 and I know now that the load on the point was 

 excessive (see Fig. 12). 



NO. 2705, VOL. 108] 



Figs, ii, a, b, c, and 12. 



The natural edges of splinters of diamond offer 

 sharper points than can be attained by grinding, and 

 I believe that the finest ruling could be made with 

 such splinters, properly mounted, if they could be 

 applied with a sufficiently light load. 



Many orders of plants form hard points, which, 

 according to Bentham, should be called "thorns" 

 when developed from the wood, and " spines " or 

 "prickles " when products of the epidermis. 



These points are often said to be " as sharp as 

 needles," and at various times I have examined a 

 great variety of them in order to determine their 

 real dimensions. Figs. 6-10 are traced from photo- 

 graphs, but only in the barbs of the spines of some 

 cacti have I found any natural points which 

 approached the sharpness of a needle. As a rule 

 "thorns" are much blunter than, "spines" or 

 "prickles." A. Mallock. 



9 Baring Crescent, Exeter, August 16. 



Biological Terminology. 



I SUSPECT that there are many others of the rank 

 and file like myself who have followed this corre- 

 spondence and feel, like a man who is a bad guesser 

 of riddles, that there is somewhere in the questions 

 asked by Sir Archdall Reid a "catch," and cannot 

 yet see it. Of course, he is too busy and earnest a 

 worker in science to ask mere riddles, and many 

 would be thankful for a concise statement of what 

 has been gained so far. The leading biologists have 

 held aloof lately, and the physiologists seem dis- 

 inclined to answer the appeal made to them. Is this 

 because Sir Archdall Reid has convinced both these 

 groups, or because they are indifferent to the issues 

 raised, or because they are waiting for them to be 

 put explicitly and some proposals made? 



Sir Archdall Reid is liberal enough to allow us to 

 give up for the moment the familiar examples of a 

 "head" and a "scar," and even the blacksmith's 

 arm. The last of these is too imaginary a case to be 

 of much use, for no one, so far as I know, has pro- 

 duced a series of, say, twent}' generations of black- 

 smiths, male and female, and demonstrated the effect 

 or non-effect of the special use of the blacksmith's 

 muscles on those of his clerical descendant. I am 

 sure he will allow me to bring forward a simpler and 

 lowlier example which appears in a book written by 

 me and reviewed in Nature of June 2, p. 419. 

 It is chosen from the mode of arrangement of the 

 hair on the ventral surface of the domestic horse's 

 neck, and I have contended that certain patterns 

 found here are inherited and produced by the 

 frequently repeated stimuli of friction in draught 

 horses due to the collar. It may, I think, be 

 granted : — 



(i) That the domestic horse is descended from a 

 wild form of the Equidae, and it shares with the 

 zebra, kiang, and onager a uniform slope of hair from 

 the lower jaw to the chest. 



(2) That numerous and varied patterns are found 

 in place of this " normal " slope in a certain large 

 number of horses to-day, and these are attributable 

 to the friction of the moving collar through many 

 generations. The inference here may be wrong or 

 right, but this does not affect the bearing of the 

 example on Sir Archdall Reid's questions to biologists 

 as to what they mean when they speak of "acquired 

 characters." It is legitimate, therefore, to speak of 

 a horse in respect of this character as "normal " or 

 "abnormal," the latter having patterns or reversed 

 areas of hair, the former none. The normal horse 



