60 



^ KNOWLEDGE 



[July 27, 1883, 



CHOLERA. 



(Preventive.) 

 To the Editor of Knowledge. 



THE progress which medical art combined with sanitary 

 arrangements have made fince 1866, has greatly 

 diminished the dread of this epidemic. Yet forewarned, 

 forearmed ; for not to be watchful against danger is fool- 

 hardy. The most important thing is not to be afraid of 

 it, since terror, fussing, and worry destroy more than 

 disease. Pay very strict attention to body and home 

 cleanliness, diet, and daily exercise, with avoidance of 

 heavy drinks, greasy substances, and fruits. 



In my large London parish the following proved most 

 successful : — E^xperience has taught me that more depends 

 upon pure water, boiled (best) or filtered, than anything 

 else. A simple filter (costing a few pence) is made out of 

 a common 6-in. garden pot plugged with sponge ; at the 

 bottom a layer of 1 ?, iru of powdered wood or bone-charcoal, 

 then 2 in. of silver sand, and on top li in. clean washed 

 small pebbles. Renovate every eight or ten weeks. Or 

 Atkins' (62, Fleet-street, London) splendid carbon filters, 

 2s. 6d. to £2. 2s. ; or Lipscombe's, 233, Strand. If 

 diarrhcea or pains come on, the following reliable Indian 

 remedy, which has been very successful during previous 

 visitations in London, and which your chemist can make 

 for 6d. to 7d. — and taken within a quarter of an hour — 

 will prove very beneficial : — 



Sp. pro Mist. Crette jss. 



Conf. Aromat. jiss. 



Tinct. Catechu 5iss. 



Tinct. Opii , m.xx. 



01. Cinnam m-^j- 



Aquas Ad. Jvj. 



Fiat Mist. 



Adults 3 table-spoonfuls every 3 or 4 hours. 



Ages 7 to 16 ... li ,, ,, ,, 



„ 4 to 7 3 teaspoonfuls ,, ,, 



„ 3 to 4 2 



„ 1 to 2 1 



„ Under 1... J 

 (Beef tea, mutton broth, arrowroot, boiled rice.) 



Always keep some of the mixture in store, also a bottle 

 of Dr. Collis Browne's C'hlorodyne. If any complications 

 arise, without a moment's delay call in your doctor. I hope 

 what formerly proved so efficacious may be again beneficial 

 to my fellow-creatures. — A. Styleman Herrixg, Vicar of 

 St. Paul's, Clerkenwell, London. 



The Edison Electric Light. — The Criterion Theatre, 

 Piccadilly, is to be lighted throughout with about six 

 hundred Edison incandescent lamps. The generating plant 

 will comprise two Armington A- Sim's horizontal high-speed 

 engines, having 13" cylinder by 13'' stroke, running at 275 

 revolutions per minute, and driving on to counter shafting, 

 from which will be run four Edison L dynamos, each 

 capable of sustaining 150 16-candle lamps. Each engine 

 will be capable of indicating 84 h.p., at 70 lbs. pressure, 

 and either engine will thus be able to take the whole load 

 in the event of accident or repairs to the other. The wiring 

 will be arranged on eight circuits, each under independent 

 control by means of a specially designed regulator, capable 

 of lowering or raising the brilliancy of the lamps from 

 normal candle power to nil The contract for the work, 

 which will be carried out by the Edison Electric Light 

 Company, stipulates for the completion of the installation 

 by the beginning of September. 



" Let Knowledge grow from more to more." — Alfred Texntson. 



Only a small proportion of Letters received can possibly be in- 

 serted. Correspondents must not be offended, therefore, should their 

 letters not appear. 



All Editorial co-mmunications should be addressed to the Editob op 

 Knowledge; all Business communications to the Publishees, at the 

 Office, 74, Great Queen-street, W.C. If this is xot attended to, 



DELAYS ARISE FOE WHICH THE EDITOR IS NOT RESPONSIBLE. 



All Remittances, Cheques, and Post Office Orders should be made 

 payable to Messes. Wymax & Sons. 



The Editor is not responsible for the opinions of correspondents. 



No COMUrNICATIONS ARE ANSWERED BY POST, EVEN THOUGH STAMPED 

 AND DIRECTED ENVELOPE BE ENCLOSED. 



LARGE SUN-SPOT. 

 [880] — It may interest " Excelsior " (letter 866, p. 29) to learn 

 that I measured the fine spot — or rather group of spots — -to which 

 he refers, on June 30, and that the total superficial area of the sun 

 involved in that stupendous disturbance amounted to 1,837,310,200 

 square miles ! The superficial area of our o^vn world, it may not 

 be unworthy to note, is only 197,000,000 square miles. 



A Fellow of the Eoyal Astronomical SoctEir. 



GEOMETRICAL AND REAL PERSPECTIVE. 



[881] — It is the practice of artists to exaggerate the size of 

 distant objects. Imagine, for example, in a landscape, a distant 

 hill with a lake in front of it, and a man walking across the fore- 

 ground so that the observer sees the collar of his coat in a line 

 with the top of the hill, and the end of his coat-tails with its base ; 

 the artist will probably show the top of the hill above the man's 

 hat, and the base coirespondingly low. TVlien the sketch is com- 

 pared with a i:)hotograph, it looks correct, but the photograph makes 

 the hill look too small. The following is suggested as a likely 

 explanation : — 



The camera takes its picture in true geometrical perspective, 

 except as regards slight errors due to the lenses, and differences of 

 focus. By using a camera with a small hole, instead of lenses, 

 these may be avoided. In the camera the man's coat takes the 

 same height on the screen as the hill does, but still, in the photo- 

 graph, the hill looks too small. On looking at a picture or 

 photograph, the eye has not to alter its focus, and can see two 

 objects supposed to be at different distances simultaneously, and 

 sharply defined. Thus, on looking at the photograph, the coat 

 covers the same height on the retina as the hill does ; but in the 

 case of the picture the hUl corresponds, perhaps, to the man's 

 coat, hat, and other kind of garment, "which, by-the-bye, the artist 

 will have shown bright red for the sake of the blue and yellow 

 of the picture. In looking at the real scene, the eye has to adapt 

 itself to the different distances, as it cannot look at near and remote 

 objects at the same time. On looking from the man's coat to the 

 hill, it alters its focus ; and this it does, not like a lens camera, but 

 by increasing the focal length of its lenses by flattening the crystal- 

 line lens. A result of this is, that the image of the hill on the 

 retina is enlarged, and occupies more height than the man's coat 

 did when in focus, justifying the artist's practice. 



The effect of this action of the eye is easily seen by holding a 

 pencil, or anything suitable, between the eye and a distant object, 

 and looking from it to the pencil and back quickly. The object 

 appears to change its size. By holding the pencil so as to subtend 

 the same angle as some dimension of a distant object of striking 

 colour, and then, having previously looked at the object till the eye 

 is somewhat fatigued, looking at the pencil, the images on the 

 retina may be compared, as the object will still be visible in com- 

 plimental colours. Jas. Swixbuexe. 



SINGULAR PHENOMENON. 



[882] — A short time ago, I was pricking out some annuals on a 

 flower-bed, on which some geraniums were already planted, when I 

 was surprised to see flashes of light coming from a truss of gera- 

 nium flowers. At first I thought it was imagination, but my wife 



