31 



that the Dames mentioned abore form the Com- 

 mittee, with power to add to their number, and 

 this was unanimously a^eed to. 



The Secretary stated that it was proposed to 

 have ext-ursions on alternate Thursday afternoons 

 during the summer. Lympne and Hythe, Rye 

 and Winchelsea, Dungeness and Minster Marshes, 

 Maidstone and District, Whitstable, etc., had been 

 suggested as suitable places to visit. 



The Chairman, before asking Mr. Mann to 

 deliver hi? lecture, menlioned that their President 

 was looking quite his old self again, and his 

 absence that evening was due largely to his 

 -enforced rest, and which he hoped would ci'ntinue 

 to be enfotced. It would be very unwise for him 

 to have ventured out that night, but they hoped 

 to welcome him back when it was warmer 

 (cheers). In asking Mr. Mann to give his lecture, 

 the Chairman said he thought he was very brave 

 to attempt to deliver it at all. Of all the subjects 

 -capable of being made plain nothing would beat 

 polarised light. 



Mr. Mann then gave his lecture on '* Polarised 

 light." He first referred to the two theories of 

 light, the corpuscular and the undulatory. By 

 the corpuscular it was supposed that light was 

 caused by the emission from luminous bodies of 

 exceedingly minute particles, which, falling on 

 the eye, so affected the retina as to give the 

 sensation of light. This theory had now been 

 .abandoned, and the undulatory theory was almost 

 universally accepted. By this theory light was 

 supposed to be due to wave motion found in the 

 ^' ether," a fluid which must exist not only 

 throughout space, but actually around the 

 particles of solid bodies. The properties 

 assigned to this ether were extreme elasticity and 

 extreme tenuity, and it was to various kinds of 

 undulations or waves in it that they owed the 

 manifestation of light, heat, electricity, and 

 magnetism. The operations of wireless telegraphy 

 were due to waves in this ether. Mr. Mann 

 showed that the movements or vibrations of the 

 ether were across the direction of the rays of 

 light, so that to a person looking at a star the 

 vibrations would be across the line from the star 

 to the observer. These vibrations were not simply 

 in one or two, but in all directions, e.g., if the ray 

 is supposed to be coming through the hub of a 

 bicycle wheel, the vibrations are along 

 the spokes. He then explained how on 

 mechanical piinciples all these vibrations 

 could be reduced to two directions, one 

 vertical, the other horizontal, and the 

 impression on the eye would be the same. Now 

 if by any means one set of vibrations were 

 <iuenched they would have light of quite a different 

 character, though even then they would need 

 special appliances to detect it. Such light was 

 said to be " polarised," or to have acquired 

 '* sides." This peculiar character could be given 

 to light by reflection from glass or polished 

 surfaces, or by causing it to pass through certain 

 crystals which, owing to their structure not being 

 uniform, caused any ray of light to be split into 

 two in passing through them. These twD lays 

 on emerging had acc^uired " sides/' and if they 

 attempted to pass them through a similar crystal 



they find their properties different from those of 

 ordinary light. The contrivance for giving these 

 " sides " to ordinary light was called the " polariser," 

 that for detecting the " aides " was called the 

 " analyser," but the two were interchangeable. 

 The lecturer polarised light from the lantern 

 by glass plates and analysed it by a 

 Nicol's prism made of Iceland spar 

 after which be showed some beautiful colour 

 phenomena. With the polariser and analyser 

 crossed so as to give no light on the screen, he 

 introduced between them a thin plate of selenite, 

 perfectly colourless, and immediately there 

 appeared on the screen a band of brilliant red. 

 On the analyser being turned a quarter-way 

 round there appeared a band of green, and he 

 next showed, by a double image prism, by which 

 these t wo colours could be produced at once and made 

 to overlap, showing that the colours were truly 

 complementary. This effect was produced by the 

 selenite splitting up the light into two parts, 

 which traversed the selenite with different velo- 

 cities, and on emerging one was half a green wave 

 length behind the other, thus causing the 

 destruction of green and the appearance of its 

 complementary colour. A considerable number of 

 designs were shown made in selenite of different 

 thicknesses, and therefore giving different colours, 

 and in each ca^e a quarter turn of the analyser 

 caused every colour to change to its complement- 

 ary. All crystals had similar effects upon light, 

 and also most oiganic substances, and an ordinary 

 quill pen flattened out gave beautiful colours. Then 

 a piece of granite ground very thin was placed 

 in the stage and again beautiful colours resulted. 

 The structure was not uniform. It was, he said, 

 bymeansof polarised light that rubies and precious 

 stones can most easily be tested, for it revealed at 

 once the peculiar structure. All bodies under 

 strain or pressure become unevea in structure, but 

 it is difficult to detect this unevenness. A piece 

 of glass was placed in the stage. As no colour was 

 produced it was evidently uniform ; but on being 

 subjected to pressure by screws it immediately 

 began to show colour, and as the pressure was 

 increased the colours became more brilliant. The 

 lecturer concluded by saying that he had 

 confined himself to experiments made by 

 plane polarised light. The effects produced by 

 circularly or elliptically polarised light were still 

 more wonderful. He wished the audience to 

 remember that not one of the cryst-als or objects 

 he had used had a trace of colour in it — that all 

 colour was produced from white light by i;he 

 destruction of colour— and that polarised light 

 was of great value in revealing to them the 

 wonderful structure of crystals and other sub- 

 stances. 



In proposing a vote of thanks to Mr. Mann, the 

 Chairman said he had accomplished his difficult 

 task, and had not only crowded into a single 

 lecture what many would have put into two, but, 

 by his felicitous language and thorough know- 

 ledge of the subject, combined with the capacity 

 for making others understand it. he bad given 

 them a most enjoyable lecture, and had been ably 

 seconded by Mr. Lander and Mr. Whalley. 



The vote was carried with acclamation, and Mr. 

 Mann briefly responded. 



