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unbroken ; yet if the same wave passes between a number of piles it is broken 

 into numerous minute waves, which take various directions and destroy or 

 augment each other. The very minute light waves are similarly dispersed 

 by very minute apertures, such as the spaces between finely-ruled lines, and 

 instead of a pencil of rays travelling in a straight line, it is split up into a 

 fan of light rays. Two such fans from two adjacent apertures overlap each 

 other, and hence from a pair of luminous points, the interspaces between 

 adjacent lines say, rays reach the eye by paths of different lengths ; when 

 the difference in the length of the path is half a wave length, there is dark- 

 ness ; when a whole wave length, light ; as the waves of the white ray differ 

 in length a series of spectra result. In order to see the object as it really is, 

 it would be necessary to rectify all the paths of all the light rays, which is 

 as impossible as it would be to put together a wave after it has been broken 

 into wavelets by the piles of a pier. As an example of this phenomenon, 

 Barton's buttons may be instanced. A smooth gold button has a metallic 

 appearance, which we all know ; rule such a button with fine lines, six or 

 eight thousand to the inch, and it appears like a diamond — that is due to 

 diffraction spectra. If it were possible to re-collect all the rays dispersed by 

 the ruled button, and bring them back to the path which they would have 

 taken had the button been unruled, it is assumed that the metallic appear- 

 ance of the button would be restored. These are not matters of theory, as 

 Mr. Smith seemed to think, but matters of fact. When we have to deal 

 with diffracted light it is most difficult or impossible to determine the nature 

 if the obstacle producing the diffraction. It is like trying to determine the 

 shape of a pier by studying the little waves produced by its disturbing in- 

 fluence. No doubt if the conditions are identical in two cases the disturb- 

 ances are identical ; hence with a given objective, focus, and illumination, an 

 object will always have the same appearance; but alter any one, ever so slightly, 

 and a totally different structure is observed. However the condition may be 

 varied, the image produced is true, in this sense, it may always be produced 

 with such conditions ; but it is not the same kind of image as that which 

 would be produced by a larger object of the same kind ; which was, he took 

 to, Mr. Smith's idea of a true image. The subject was one in which, per- 

 sonally, he felt very much interested, and he was therefore glad to have had 

 it brought forward. He felt sure the Members would give a hearty vote of 

 thanks to Mr. Smith for his paper. 



Mr. T. F. Smith said, with regard to the remarks of Mr. Nelson, question- 

 ing his statement that Prof. Abbe's views were first published in the 

 " Microscopical Journal," he believed that he was wrong, the fact being as 

 stated by Mr. Nelson ; but as far as the wording was concerned he had care- 

 fully followed it wherever it had been necessary to make any quotations. 

 With regard to the diffraction theory, he had not expressed any opinion upon 

 it either for or against, but what he pleaded for was that they should be at 

 liberty to judge of a thing by what they saw. 



The President said that if the diffraction of light was ignored altogether, 

 then, of course, he was out of court entirely. 



Mr. Priest read an extract from a letter from Mr. Harris, of Cardiff, 



