November 2, 1899] 



NATURE 



he looks at the matter entirely from the enthusiastic botanist's 

 point of view. 



The Government of India does not wish every Indian forest 

 officer to be a botanist. It is desirable that every now and then 

 one of them should take up the subject as a speciality, but it 

 would be disastrous if all took that line. I have no hesitation 

 in saying that as soon as a forest officer takes up botany as a 

 speciality he is, rare cases excepted, likely to become an in- 

 different forest officer. The ordinary officer of that class has no 

 time for special botanical study. 



Forestry' is perhaps not a science in itself, but an industry 

 based upon various branches of science, amongst which botany, 

 geology and entomology are the most important. The forest 

 nicer cannot be an expert in each of these. To demand such 

 ,t thing would be just as unreasonable as to demand that a 

 medical man should be an expert in chemistry. The one is as 

 impossible as the other ; to become either takes practically a 

 life-time. With the enormous growth of the several branches 

 of science a very minute specialisation has become an absolute 

 necessity, since only a very small fraction of men can be classed 

 as geniuses, while the rest must be rated at the average capacity 

 of the human race. The student of one branch must depend on 

 the work of students in other branches. Thus the forester, 

 instead of being the assistant of the botanist (as Sir G. King 

 seems inclined to demand), must rely on the professional 

 botanist for all the finer and more intricate problems of botany. 

 All he requires is to acquire a sufficient knowledge of botany, 

 so that he may utilise what the professional botanist tells him. 

 For more he has no time, because he has to attend to quite 

 another class of business. The Indian forest officer is an estate 

 manager on a large scale ; he must manage his estates in such a 

 manner that they yield the largest possible amount of useful 

 produce with the least possible outlay. For that end his time 

 is taken up by sylvicultural and administrative duties, leaving 

 but little of it for the special study of any of the branches of 

 science upon which systematic forest management is based. 



No doubt many of the pioneers of Indian forestry were 

 botanists, but by no nieans all. Take, for instance, the pro- 

 tection of the forests against fire, a matter to which Sir G. 

 King gives prominence. He himself states that Lieutenant 

 (now General) Michael was the first who was successful in this 

 direction in Madras. I may add that, as far as Central and 

 Northern India are concerned. Colonel Pearson was the first to 

 introduce successful fire conservancy. And yet neither of these 

 two gentlemen will, I feel sure, claim to be a great botanist. 



Sir D. Brandis, to whom, as Sir G. King points out, we 

 owe, for the most part, the organisation of the Indian Forest 

 Department, no doubt was a botanist ; but he brought about 

 that organisation, not as a botanist, but as an able forester and 

 administrator of extraordinary energy. 



Botany is a branch of science the study of which is most 

 fascinating ; but the faculties which produce a great botanist do 

 not necessarily include those which are required to produce a 

 great administrator ; and herein lies the difficulty, in so far as 

 the Indian Forest Department is concerned. I could point 

 out more than one botanist who occupied the post of the 

 head of the Forest Department in a province, and who 

 could not possibly be counted amongst the successful forest ad- 

 ministrators of India. In nearly all these cases so much time 

 was given to botany that little — or, at any rate, not enough — 

 time remained for the proper administration of the extensive 

 Government forest estates which supply the people of the 

 country with the necessary forest produce, and over and above 

 yield now an annual net revenue of a million pounds. These 

 results would l>e most seriously imperilled if our Indian forest 

 officers were to take the line which Sir G. King recommends 

 to them. W. ScHLiCH. 



Coopers Hill, October 19. 



Dark Lightning Flashes. 



As an amateur photographer of cloud-scenes, I have taken 

 the image of the setting sun surrounded by clouds on many 

 occasions. I never remember developing a plate in which the 

 image was reversed after an ordinary rapid exposure. Light- 

 ning flashes, one would think, ought to be still more rarely re- 

 versed, if the chemical reactions of the salts in the gelatine film 

 are solely responsible for the phenomenon ; yet dark lightning 

 flashes are not infrequently visible in the developed plates of 

 a thunderstorm. 



Dr. Lockyer's interesting photographs (vol. Ix., p. 570) of dark 



NO. 1566, VOL. 61J 



flashes with bright cores suggest to my mind another inter- 

 pretation. A lightning flash (and, for the matter of that, an 

 electric spark) is doubtless a complex phenomenon. A disruptive 

 discharge of high tensional electricity through the atmosphere 

 represents, I take it, a core of rarefied (because incandescent) 

 gases surrounded by an envelope of compressed air. Mr. C. 

 V. Boys has shown (Nature, vol. xlvii. p. 420) that "a 

 wave or shell of compressed air gives rise to an image on 

 the plate in which there is a dark line and a light line within 

 it. Similarly, a wave of rarefaction must produce a light line 

 with a dark line within it." Surely we have then in the 

 lightning flash itself, when rightly illumined, the necessary data 

 for the production of an image —a bright line edged with two 

 dark lines, as represented in Dr. Lockyer's photographs. In 

 such cases the advantages of a diffused illumination of the back- 

 ground of the scape are obvious. Possibly Mr. S. Bidwell's 

 interpretation of the double flash is the correct one. 



Hove, October 21. W. Ainslie Hollis. 



It seems to me difficult to compare the photographic bright- 

 ness of the disc of the setting sun with a brilliant flash of light- 

 ning. For my part I consider that lightning flashes give us 

 every chance of obtaining photographic reversals, for they can 

 be photographed at very close distances, amounting to a few 

 hundred yards, while the rays from the sun's disc when near 

 the horizon must pass through a long range of dense atmosphere 

 which cuts off" the most actinic and therefore photographic rays. 



With regard to the second portion of Mr. Hollis' letter, the 

 illustration in my article (Nature, vol. Ix. p. 573, Fig. 6) 

 disproves rather than proves his suggestion in my estimation. 

 If, as he assumes, the core may be considered the actual spark, 

 and the outer portion the image of the wave or shell of com- 

 pressed air, then, as the latter is not so luminous as the core, it 

 ought to be best visible by reason of contrast against a bright 

 background. A glance at Fig. 6 shows that this is not the case, 

 for at c the core exists practically alone with an illuminated 

 background, while without the background at A and B it is most 

 developed. 



I cannot convince myself that the large dark flash is a double 

 one. A close examination of the negative strengthens the view 

 that it is single, and the general appearance of the ramifications 

 endorses it. William J. S. Lockyer. 



Solar Physics Observatory, South Kensington, October 24. 



A Gutta-percha Plant. 



In your issue of October 19 you report a communication 

 made to the French Academy of Sciences by Messrs. Dybowski 

 and C. Fron regarding the cultivation of Eucommia ulmoides, a 

 plant said by them to contain gutta-percha. I am naturally 

 much interested in the possibility of this interesting tree, the 

 " Tu chung " of the Chinese, becoming of economic importance, 

 as some years ago I investigated the bark and leaves of this 

 plant with regard to the peculiar cells containing a rubber-like 

 substance {Trans. Linnean Society, 1892, vol. iii., part 7). 



Gutta-percha and caoutchouc behave very similarly towards 

 many solvents ; but the fact that the contents of these cells were 

 dissolved or partially dissolved by turpentine at ordinary tem- 

 peratures, whereas gutta-percha is only soluble in hot turpentine, 

 led me to the conclusion that the contents of these cells were 

 caoutchouc. This substance is much more frequently met with 

 in the laticiferous cells than gutta-percha, which is almost 

 restricted to the natural order Sapotaceoe. Eucommia will 

 therefore, I think, be found to be a rubber, and not a gutta- 

 percha yielding plant. 



But in either case it is obvious that, with the opening up of 

 China, this plant may become of great economic importance if, 

 as seems probable from the investigations of Dybowski and 

 Fron, it is easily cultivated and propagated. 



F. E. Weiss. 



The Owens College, Manchester, October 23. 



Halo Round a Shadow. 



On a winter morning some years ago I was driving in a dog- 

 cart from the Lizard across the Goonhilly Downs whilst a dense 

 mist or cloud was matted down on the ground. 



Our heads were in bright sunshine, which formed a coloured 

 halo round the shadow of each of our heads on the mist as 

 we travelled on. Half an hour later the mist was more diff'used, 

 and we saw a white mist bow in the sky. Howard Fo.x. 



Falmouth, October 28. 



