February, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



353 



The corresponding rule for a camera focussed for a 

 finite distance is best shown with the aid of the acconi- 

 panvint; figure. If A B represents the stop or its 

 magnified image as seen in the front lens when focussed 

 on a point P, at any given distance, details at any 

 other distance will be separated in the photograph if 



iheir distance apart exceed the breadth of the cone 

 A P B at that distance. I-' or instance, a h imd a' b' 

 show the distances apart of points when the impressions 

 they produce in the photograph just cease to be 

 separate. 



I found these rules of great use during the recent 

 visit to South .\frica, and the results obtained were in 

 complete accordance with them, especially when other 

 conditions were taken into account. 



CORRESPONDENCE. 

 Green Flash at Sunset. 



I would call attention to a phenomenon of atmospheric 

 refraction frequently observed, but of which very little has 

 been published. I refer to the green flash seen in clear 

 weather as the last ray of the setting sun disappears. I 

 observed this one evening in the N. .Atlantic last month, and I 

 also saw it in the N. Atlantic in Octoljer, lytjH ; on this last 

 occasion I watched Venus setting a little later, but though the 

 planet turned red occasionally as it approached the horizon, it 

 did not flash green at the last moment. I have, however, seen 

 stars set with a green flash, but rarely. 



The phenomenon is noticed in " Knowledge" for April, 1889, 

 p. I2(), here the colour is given as blue ; in " Nature," March 

 and .A.pril, 1S90, pp. 495, 538; and in K.A.S. notices, May, igoi. 

 In these notices the writers appear to hold different views as 

 to the cause. In Elementary Meteorology, by Professor W. M. 

 Davis, p. 50, it is stated that after sunset every solar beam will 

 be broken into a short vertical spectrum ; is this the e.xplana- 

 tion ? If so, why should the last flash be green (according to 

 some accounts, of a remarkably vivid green) ? I would like to 

 see the matter worked out. 



J. P. Maclear. 



Chiddingfold, igth Dec, 1905. 



[Admiral Maclear's letter opens up the subject which has for « 

 very long time formed a topic of discussion, especially among 

 voyagers at sea. We have always held the belief that the pheno 

 menon is solely due to the optical effect, so well known, of a com- 

 plementary colour appearing on the withdrawal of any very brightly- 

 coloured object. If ycu look at the sun and then close your eyes, a 

 brilliant green image will be seen. — Ed. J 



The late Prof. Howes. 



We are requtsted to announce that the endowment fund nov 

 being raised for the family of the late Professor (■. H. Howet- 

 F.K.S., will be closed shortly, and all intending contributor; 

 are asked to send their contributions without delay to tht 

 Treasurer, Mr. Frank Crisp, at 17, Throgmorton Avenue. 

 Londor, E.C. 



Influenza and the Weather. 



To the Editor of " K.nowledge and Scientific News." 



Sirs, — In your issue of November there was a most in- 

 teresting article from the pen of Mr. A. H. Bell re the above 

 subject. It is a subject of the utmost importance to the 

 public, for a ravage of influenza has in its tiny germ as deadh- 

 a killing factor as shot and shell in a big war. That Mr. Bell 

 is perfectly right, most doctors who read his article will per- 

 fectly agree. Most will also see in the much abused east wind 

 one of Nature's best antiseptics, .so that instead of being the 

 sower of disease we would rather look upon it as a trying but 

 invigorating friend. True, it may be a dread to the very young, 

 the very old, and to the feeble and confirmed invalid ; but to 

 the great mass of mankind it certainly does not deserve all 

 the scape-goatish epithets flung at it. 



From personal e.xperience I can corroborate every word 

 written by Mr. Bell, for it has been my fortune, or misfortune, 

 to contend with more than one attack of severe influenza 

 whilst practising in England. 



In a large country practice I found that in no case did 

 influenza spread e.xcept by the contact of the healthy with the 

 affected. 



Farmers and their wives had it first, then their families, 

 lastly their servants. Time after time noticing that those 

 who did the marketing were the first to be struck down, con- 

 vinced me that influenza chiefly spread by contact. 



In the Blue Book which Dr. Franklin Parsons drew up for 

 Parliament when the first severe visitation of influenza some 

 20 years ago visited Great Britain, there is a report of mine 

 bearing out the opinions so clearly set forth by Mr. Bell in his 

 article in •■ Knowledge." In a conversation I had with Dr. 

 F. Parsons he, too, was fully convinced that climatic changes 

 had little to do with the spread of influenza, and in coming to 

 that conclusion he had his own observant mind guiding him, 

 as well as the e.xperience of others. 



I am, dear Sir, 



Yours truly, 

 T. PicKTHORN Thomson, 

 C. S. Duplex, 

 Suez, Egypt. 



Venom of Spiders. 



To THE Editor of " Knowledge." 



Sirs, — I have read with great interest Mr. C. A. Michell's 

 letter relating to the venom of spiders. I have always had 

 great interest in this subject, and have studied it for many 

 years. 



When I was a boy at school I was very fond of natural 

 history, and used to spend much of my spare time collecting 

 specimens. One day I succeeded in catching a very large 

 black spider, but unfortunately I did not notice the variety. 

 Having nowhere to put it, I placed it in a bo.x containing two 

 frogs. To my surprise ne.xt morning I discovered that one of 

 the frogs was dead and the other very torpid, and succumbed 

 in a few minutes. The spider was quite lively, and when dis- 

 turbed with a straw attacked it furiously. 



Happening to mention this fact to our old gardener, who 

 had a great reputation in the village for his knowledge of wild 

 things, he declared that the bite of a spider was fatal to all 

 small creatures, but only during a certain part of its existence. 

 Wishing to prove this I tried the next night with another frog. 

 In the morning the frog was dead, but the spider had un- 

 fortunately escaped. I tried several times after this, but 

 these were the only cases in which the frogs were killed. I 

 shall be much obliged if any of your readers can inform me to 

 which variety my spider belonged. I have never since seen 

 one like it. The points that struck me most about it were its 

 great size, its broad, smooth back, and the orange stripe along 

 the lower side of its body. 



E. T. Z. D. 



The Elms. 



