284 



KNOWLEDGE. 



[December 1, 1900. 



for communicating between ships, or ships and the shore, 

 at short distances. 



By the use of reflectors it is possible to project the 

 electric waves in an almost parallel beam, which will 

 have no effect upon any receiver not lying in its course, 

 whether this receiver be syntonic with the waves or not. 

 This, as Signor Marconi has pointed out. would enable 



Fia. 8 



Mast at tUe Suiitli Furelaud. 



several forts, hill-tops, or islands to communicate with 

 each other in war time, without any fear of the enemy 

 tapping or interfering with the signals, for, if the forts 

 were on a small height, the beams could easily be 

 directed so as to pass over any position that might 

 possibly be occupied by the enemy. 



In some experiments, made over a distance of 

 one and three-quarter miles, Signor Marconi ob- 

 served that quite a small movement of the reflector of 

 the transmitting instrument was siifficient to stop the 

 reception of the signals by the receiver. The zone, 

 within which the receiver had to be placed for a given 

 position of the transmitting reflector, not being more 

 than about 100 feet in breadth. 



" There exists," says Signor Marconi, in his paper read 

 in March, 1899, before the Institution of Electrical 

 Engineers, " a most important case to which the reflector 

 system is applicable, namely, to enable ships to be warned 

 by lighthouses, light-vessels or other ships, not only of 

 their proximity to danger, but also of the direction from 



which the warning comes. If we imagine that A is a light- 

 house, provided with a transmitter of electric waves, 

 constantly giving a series of intermittent impulses or 

 flashes, and B a ship provided with a receiving apparatus 

 placed in the focal line of a reflector, it is plain th-it, 

 when the receiver is within range of the oscillator, the 

 bell will be rung only when the reflector is directed 

 towards the transmitter, and will not ring when the 

 reflector is not directed towards it. If the reflector is 

 caused to revolve by clockwork, or by hand, it will 

 therefore give warning only when occupying a certain 

 section of the circle in which it revolves. It is there- 

 fore easy for a ship in a fog to make out the exact 

 direction of the point A, whereby, by the conventional 

 number of taps or rings, she will be able to discern 

 either a dangerous point to be avoided or the port or 

 harbour for which she is endeavouring to steer." 



Marconi's apparatus was installed la.st month on board 

 " The Princess Clementine," one of the Belgian boats 

 carrying on the passenger service between Dover and 

 Ostend. It is stated that the distances covered exceeded 

 eighty miles, and that the apparatus is shortly to be 

 supplied to other vessels belonging to the Belgian 

 Company. 



There is a difficulty which will have to be overcome if 

 the system is to come into extensive use, and that is 

 the interference of simultaneous messages coming from 

 different stations, all of which would affect all receivers 

 within range, with the result of making the messages 

 unintelligible. 



Professor Lodge hcis devised and patented some 

 interesting forms of syntonic transmitters, but, as far as 

 I am aware, he has not yet succeeded in transmitting 

 syntonic messages over any considerable distances. A 

 syntonic radiator is necessarily one which produces per- 

 sistent oscillations instead of having them damped out 

 almost immediately. Now this damping out means that 

 the ether rapidly ta.kes up the vibrations, so that making 

 a radiator syntonic means making it feebler. 



It is stated that Signor Marconi has now succeeded 

 in devising apparatus by which syntonic messages have 

 been successfully transmitted to distances of over 30 

 miles, but as the patents are not yet completed I am 

 not able to obtain any information as to the methods 

 employed. 



If he can do this it will prevent the interference 

 above referred to, as the receiver at any station will 

 respond only to the messages intended for it. 



Figures 2. 4, 6. 7, and 8 are reproduc'ed by kind 



permission from Signor Marconi's paper in the " Journal 



of the Institute of Electi-ical Engineers, ' Vol. 28, 1899. 



Eeeatum. — Article IV., Knowledge, October, liluu, p. 235 



column 2, line 7, for " 10'7 centimetres," read " lU"' cm." 



iilicrosropg. 



By John H. Cooke, f.l.s., f.g.s. 



A convenient means for testing the optical qualities of objec- 

 tives is very desirable. It is essential that the objects used for 

 this purpose should be most carefully prepared and properly 

 mounted. For low powers the proboscis of the blow-fly is 

 usually recommended : also the scales of Initterflios. These 

 objects should appear distinct in detail, flat, and free from 

 marginal colouring. For medium powers stained micrococci, 

 bacteria, and starch giaiiis are useful. For high powers, 

 especially immersion objectives, various diatoms are used, such 

 as Navii-ttUi hji-a, Pleurosigma anguhitum and Aiuphipleura 

 pellucida. 



Those who do not possess a turntable will find a slide centerer 

 au excellent substitute. This piece of ajiparatus may be readily 

 prepared as follows: — Rule a rectangle the size of an ordinary 



