Feb. 26, [885] 



NA TURE 



389 



to your readers that a screen, whose valuable properties seem 

 even now to be scarcely at all known, should be noted in your 

 columns. 



It simply consists of a sheet of French tracing-paper, of a kind 

 which possesses a remarkably dull, non-reflecting surface. With 

 this screen and only an oil-lamp lantern, it is quite easy to show 

 pictures well to a couple of hundred people in a room fairly 

 well lighted — sufficiently lighted indeed to enable note-taking 

 or reference to books to be accomplished with perfect ease — 

 provided that extraneous lights are not placed behind the 

 screen. 



It was to Mr. George Smith, of the Sciopticon Company, that 

 I was indebted, four years ago, for the knowledge of this fact ; 

 which, with considerable lantern experience, I scarcely knew 

 how to believe, until I had myself verified it. 



At present, however, the tracing-paper cannot, I believe, be 

 obtained more than three to four feet square. 



Charles J. Taylor 



Toppesfield Rectory, Essex, February 17 



Fuller s Earth as a Filter 

 Where the fuller s earth is dug from the Bedfordshire green- 



sand it is held in much repute for its efficacy in removing im- 

 purities from turbid water. 1 In addition to the other uses to 

 which it is here applied, dealers take it around through the fen 

 countries, and dispose of it for clarifying the peaty water,' 2 often 

 the only supply obtainable in those districts. 



I shall esteem it a favour on the part of the readers of Nature 

 residing on the Greensand or Oolites of the southern counties to 

 notify if these filtering properties of the Bedfordshire fuller's 

 earth are in any way unique — in so far as they appear withheld 

 from that of other places? — as at Reigate, Bath, &c, where 

 fuller's earth is known to them to be dug. 



Bedford, February 23 A. G. Cameron 



The Boomerang in India 



In Gustav Oppert's work " On the Weapons, Army Organisa- 

 tion, and Political Maxims of the Ancient Hindus," the boome- 

 rang is mentioned as being among the weapons, especially in 

 Southern India, and made of various materials — iron, ivory, 

 and wood. Are any specimens to be found in our museums 

 here, or would any private persons who may happen to possess 

 any, kindly allow me to inspect them ? 



Arthur Nicols 



THE CAMERA 



OBSCURA 

 WORK'' 



IN TORPEDO 



A T the time of the last Austro-Italian war, in 1866, the 

 •**■ Austrian Government made the greatest efforts to 

 protect its harbours from an attack of the Italian fleet. 

 Torpedoes were placed in great numbers in the harbours, 



and the greatest vigilance was enjoined on all the com- 

 mandants of such places. 



The accompanying illustration represents a novel appli- 

 cation of the camera for use at the observing or firing 

 post of a party belonging to the military telegraph. The 

 torpedoes are placed along certain concentric lines, very 

 close to each other, and at a certain depth below the sur- 



face of the water, no sign of their presence being appa- , cates the exact position of each torpedo ; these points are 

 rent. A metallic wire connects each of them with a post all numbered, each number corresponding with that on a 

 of observation situated on the coast at a point sufficiently particular key of a keyboard. To press one of these keys 

 elevated to permit of the port being seen. According to with the finger is sufficient to place the corresponding 

 well-known optical laws, an image of the port is formed 

 on the glass. Black points marked on that image indi- » C»/ : Miy., February. 18S5. 



A brief account of the method in u; 

 1 From La Nature. ! shire and Lincoln will appear shortly. 



fen districts 'of Cambridge- 



