454 



NATURE 



[February 7, 1918. 



show the local disturbing forces, which are discussed 

 in pp. 28-31. Amongst the largest disturbances are 

 those in Stewart Island and near Invercargill and 

 Dunedin. Prof. Marshall contributes in pp. 63-64 some 

 remarks on the geological character of the disturbed 

 regions. His conclusion is that "while in each case 

 of magnetic irregularity it is possible to point to some 

 unusual geological feature, these are in no instance 

 the most marked feature of that kind in the country, 

 and those localities where such feature is most pro- 

 nounced show no unusual magnetic characters." As 

 Dr. Farr says himself, there is room for considerable 

 further observational work in the disturbed districts. 



Two supplementary pages give particulars of ob- 

 servations made in March and April, 19 16, at ten of 

 Dr. Farr's stations by Mr. W. C. Parkinson, once of 

 Greenwich and Eskdalemuir Observatories, now ob- 

 serving for the Carnegie Institution of Washington. 

 These serve a useful purpose in showing the changes 

 that have occurred since the epoch of the survey. 



Transport is still a serious difficulty in parts of New 

 Zealand, and the work had to be carried on in the 

 spare time which his other important duties left at 

 Dr. Farr's disposal. He is to be congratulated on 

 having brought to a satisfactory conclusion an arduous 

 piece of work, which adds substantially to our know- 

 ledge of terrestrial magnetism in the southern hemi- 

 sphere. It is satisfactory to notice that the work had 

 the active support of the New Zealand Government, 

 and that the printing was done, and satisfactorily 

 done, at the Government Press. C. Chree. 



PARASITES OF CROPS AND CATTLE. 



\TERY striking data as to the extent of the loss of 

 ^ crops occasioned by diseases of parasitic origin 

 are contained in the paper on economic mycology 

 read by Prof. M. C. Potter at the Newcastle meet- 

 ing of the British Association (1916), and since pub- 

 lished in the Journal of the Royal Horticultural 

 Society (vol. xlii., parts ii. and iii.). In the year 1891 

 the loss to the German Empire upon the total cereal 

 crops was estimated at more than 20 millions sterling, 

 an amount nearly equal to one-third of the total value 

 of the crop. In the same season (1890-91) the loss due 

 to rust of wheat in Australia was estimated at 2^ 

 millions. The case of potatoes is even more notorious. 

 In Germany the loss due to disease of the potato crop 

 amounted in one year to 30 millions, and in our own 

 country it is computed that, on the average, the crop 

 is reduced by disease by at least one-third. It is esti- 

 mated that in Northumberland and Durham about 

 half the crop of swedes and turnips is destroyed in 

 average years by parasite attacks. Ivosses of timber 

 also are very serious, and probably amount to one- 

 third of the whole. Other crops, such as tea, rubber, 

 hops, and every kind of fruit, greenhouse, and garden 

 crops, all pay a heavy toll to fungus diseases. A 

 plea is entered for greater encouragement by botanists 

 to the prosecution of research in phyto-pathology and 

 for the wider treatment of the fungi in ordinary 

 botanical courses, especially from the point of view 

 of their work in Nature. The scope of the problems 

 awaiting solution in this field is abundantly illus- 

 trated, and appreciation is expressed of the increased 

 attention and support given to it in recent years by 

 Government departments and other institutions, al- 

 though further provision is still urgently necessary. 



A ref>ort on investigations into the cause of worm 

 nodules (Onchocerca gibsoni) in cattle by Messrs. C. G. 

 Dickinson and G. F. Hill has been issued as a Bulletin 

 (C. 9341) by the Government of the Commonwealth 

 of Australia. Two series of experiments were carried 

 out in the Northern Territory with calves from 



NO. 2519, VOL. 100] 



nodule-free districts of Victoria. Calves grazing on 

 high, dry ground along with infected cattle became in- 

 fected within eight months of arrival, whereas similar 

 nodule-free calves did not become infected during the 

 same period when enclosed in an open pen with con- 

 crete floor within 30 yards of a paddock within which 

 alTected cattle were depastured, although exposed 

 to the attacks of winged and apterous Arthropoda. 

 The results, while not revealing an intermediary host 

 of the parasite causing Onchocerciasis in cattle, have 

 definitely excluded certain species that were regarded 

 as possible vectors, namely, Lyperosia exigua, Sto- 

 moxys calcitrans, Tabanus mastersi, T. nigritarsis, 

 Boophilus australis, and any purely aquatic forms 

 other than those possibly found in the bore-water. 

 Various common species of mosquito, it is thought, 

 may also be excluded. Wild swamp buffaloes were 

 not affected, whereas wild Zebu cattle and domestic 

 cattle grazing on the same country are invariably 

 affected. 



LIGHT AND VISION.-^ 



THE old Greek philosophers who did so much think- 

 ing and so little experimenting had queer ideas 

 about light and vision. Empedokles, who died about 

 420 B.C., considered it necessary to record the fact that 

 darkness is not a real thing, but privation of light ; 

 and that the moon shines with reflected light, but he 

 thought that the sun is the primary fire of the light 

 of the sky reflected in a crystalline spheroid. Demo- 

 critus, who died about 370 B.C., held that vision was to 

 be explained by emanations or exceedingly thin husks 

 or films which were continually being detached or 

 thrown off from the surface of bodies, and that they 

 penetrated into the sense-organs through fine passages 

 or pores. We admit this in the case of taste and of 

 smell. These ghost-like forms or images were called 

 eidola (eiScoXa), whence we have the word idol (a very 

 different kind of image from those considered in optical 

 books), and were supposed to be ever passing from the 

 object to the moist and receptive surface of the eye 

 straight into the mind. Aristotle, who died about 

 325 B.C., seems to have objected to some of the earlier 

 theories. He scarcely alludes to light and vision in 

 " De Physica," but there is some reason to suppose 

 that a treatise by him on optics has been lost. More 

 than two centuries later Lucretius, the scientific poet, 

 discussed the theory at great length in the 

 fourth book of " De Natura Rerum." He 

 used the expression simulacra quasi inembranae, 

 resemblances like films, peeled off from the upper sur- 

 face of things, flying hither and thither on one side 

 and the other through the air. Simulacra was also 

 used for ghosts, and he goes on to explain how they 

 terrify us in sleep. He also attempted to explain the 

 action of curved mirrors, of the distance of the image 

 behind a mirror, and why the theory does not work in 

 the dark. 



The schoolmen in the Middle Ages tried to follow 

 Aristotle as closely as they could, but matter and form 

 probably did not mean to them what they meant to 

 Aristotle or to us. The eidolon was still used, but 

 the expression had lost its materialistic signification. 

 At the end of the sixteenth century men began to shake 

 off dogmas of authority, to think for themselves, and 

 to follow inductive lines of reasoning. 



We may perhaps flatter ourselves that in our branch 

 of applied optics we are not trammelled by fundamental 

 theory, and that if the corpuscular hypothesis of light 

 came back again into fashion next week to replace the 

 undulatory theory, as the electron has pushed aside the 



1 From the presidential address delivered b-fore the Illuminating Engineer- 

 ing Society on December i8, 1917, by A. P. Trotter. 



