December i6, 1920] 



NATURE 



503 



substances called acids by the chemist, surely does not 

 need a choice of seven brand-new names. In fact, one 

 Faraday did some very important work indeed in the 

 subject generations before the modern hydrophobic 

 school, with its inveterate aversion to "anything wet," 

 had arisen. Frederick Soddy. 



The Stereoscopic Appearance of Certain Pictures. 



Dk. Kdkidck (jKekn's explanation on p. 375 of 

 Nature of November 18 does not go quite far enough. 

 It is true that a picture in correct drawing and per- 

 sjxjctive will be correct only for one eye, but the eye 

 must be situated at a certain point which is geo. 

 metrically defined by the elementary rules of perspec- 

 tive, k photograph taken by a pin-hole camera or 

 with any good lens is in true perspective. It should 

 be viewed at the same angle as that with which it 

 was taken ; in other words, it should be viewed from 

 a distance equal to that of the pin-hole (or a certain 

 point in the lens combination) from the plate. 



Photographs and illustrations are generally viewed 

 at too great a distance. With an angle of view of 

 55° the eye should be at a distance approximately 

 equal to the longest dimension of the picture. If this 

 is considerably less than 250 mm. to 350 mm. (say 

 10 in. to 14 in.) a lens must be used, not for magni- 

 fication — which is a disadvantage in the case of 

 coarse-grained process blocks — but to enable the eye 

 to be used at appro.ximately the right position. 

 Exactness is not necessary, as the eye is so easily 

 pleased. An ordinary reading-glass may be used for 

 the illustratwl papers, but it must be held close to 

 the eye. The result is sometimes very striking. 

 Photographs of complicated instruments or of com- 

 plicated machinery in a factory, which are scarcely 

 intelligible when viewed in the ordinary way, stand out 

 almost as solid as with a binocular stereoscope. 

 Partly because photographs, illustrations, and pictures 

 are generally viewed with both eyes, and partly 

 because the distance is usuallv niuch too great, 

 .nccurate perspective representation, as a rule," pro- 

 duces no stereoscopic illusion whatever. 



.\. P. Trotter. 



Ajaccio, Corsica, December 3. 



Luminosity by Attrition. 



Thb following worl<shop observations may throw 

 some additional light on luminosity by attrition, the 

 subject of recent correspondence in Nature : 



Blocks of optical glass are cut into slabs by means 

 of a soft steel circular saw, the edge of which is 

 usually charged with diamond-dust, a copious flow of 

 a lubricant such as petrol, paraffin, or soapy water 

 being employed. 



Within the block of glass at the line of contact 

 with the saw there is often visible a blue-tinted white 

 light, limited to the acting portion of the edge of 

 the saw. The li^ht is not thrown downwards in the 

 direction of motion, as in the case of a shower of 

 sparks or of an ordinary flame. 



The bluish-white light is most apparent when the 

 cutting is forced, when the saw is blunt, and when 

 an insufficient amount of diamond-powder is used. 

 When the saw Is working well the light can only be 

 seen with difficulty by excluding extraneous light. 



There is no evidence of any temperature cracking 

 over the cut surfaces, and I know of no instance in 

 which the petrol or paraffin has been ignited even 

 w h'n the saw is cutting at the extreme edge of the 

 (;ln'^s in contact with the air. 



When carborundum is used in<>tcad of diamond- 

 powfler — the carborundum, however, being injected 



NO. 2668, VOL. 106] 



I with compressed air and a water 

 ! ployed — a dull reddish light appears. 



lubricant em- 

 ployed — a dull reddish light appears. It is difficult 

 I to say if this reddish light fs accompanied by anv 

 characteristic heat cracks, as the cutting action with 

 carborundum is more complicated, there being a 

 secondary side abrasion at the surfaces of the saw 

 which might obliterate minute heat-cracks. 



James Weir French. 

 .\nniesland, Glasgow, December 8. 



I Tragic Death Feint of a Snake. 



I On Sunday morning, May jo, about 10 o'clock, 

 I I noticed a common western hog-nosed viper, about 

 I 20 in. in length, basking on the lawn in the warm 

 I sunshine. I approached the serpent in company with 

 ! a friend to make some investigation of it, and only 

 ; to interfere with it enough to keep it from crawling 

 i away. The creature went through the usual feint ol 

 j being a dangerous snake that is peculiar to this 

 species, and quickly began to coil and recoil and to 

 hide its head under its body, .\fter it had done this a 

 j short time it turned on its back, but continued to 

 I writhe as though injured severely. Gradually it 

 assumed a position simulating that of a dead snake 

 j lying on its back, with its mouth completely inverted 

 ; and bleeding. This was done in such a way that the 

 i head appeared to be completely mashed or severed. 

 The exudate of blood from the entire surface of the 

 mouth was perfect. It was the most complete and 

 well-carried-out feint of a tragic death that I have 

 ever witnessed, and all without the least torture or 

 stroke of any kind from me. I only detained the snake 

 by placing my foot in front of it and turning it back 

 once at the beginning. We left the creature in this 

 apparently killed condition, only to see that it dis- 

 appeared in a very short time. 



My observation of this genus Heterodon (hog-nosed 

 viper), which is not a viper at all, has shown me that 

 it always puts up the tamest kind of bluff before 

 hiding its head, but never before have I observed this 

 complete performance with a bloody exudate from the 

 inverted mouth. I am convinced that it will not 

 often be carried out thus completely unless the condi- 

 tions of season, the weather, and the development of 

 the snake are just right. In other words, I think 

 thpt it must be a peruliaritv of some maturity of 

 RTOwth. and that the full vigour of a warm day in 

 late spring or early summer must enter into it. 



W. E. Bartlrtt. 

 Belle Plaine, Kansas, U.S.A., November 22. 



The Alkaloids of Senecio jacobaea. 



In the Notes in Nati;rb of November 4, p. 321, 

 reference is made to " Senecio jacobaea, the source of 

 the disease in sheep in Nova Scotia." It should read 

 "cattle" instead of "sheep," for although injurious 

 to sheep it has not been fatal to them as it has 

 been to cattle. The " Pictou cattle disease " has in 

 some quarters led to change of the common name 

 " St. James ragwort " to " cattle-kill " — a term 

 analogous to " lamb-kill " for Kalmia glauea and 

 K. angustifolia, supposed to be poisonous to young 

 sheep. 



The alkaloids of Senecio jacobaea were, under the 

 auspices of the Nova Scotia Institute of Science, 

 planned to be investigated by the Intc Dr. Ebon 

 MacKay. of the University of nalhoustc, on the 

 chemical side, and by Prof ' ! Moore on the 

 biological side. \ H. MacKav. 



Education Office, Halifax, Nova Scotia, 

 November 33. 



