568 Notes. 



oblique incidence correctly, because the last spectrum did not vanish till the 

 distance between the Lines or corrugations was as small as half a wave- 

 length, his arguments had been perfectly sound. 



When no diffraction spectra were formed, then the whole of the light 

 must be concentrated in the specially reflected beam, and the corrugations 

 then had no effect. It was, therefore, entirely a question of wave-length. A 

 surface roughened with pebbles might act as a perfect reflector for sound- 

 waves. The author had himself experimented with a piece of ground glass 

 silvered over the roughnesses, which acted as a reflector for dark heat rays, 

 and again, glass might be polished sufficiently finely to reflect red rays 

 fairly well, and yet act very imperfectly as regards reflection of the blue or 

 ultra-violet rays. Herschel had thought that grinding and polishing of 

 glass was of the same nature, that lumps of glass were broken out by the 

 emery with which the glass was brought into contact under pressure. His 

 own observations led to a different conclusion — viz. that polishing, as con- 

 ducted with rouge imbedded in pitch, or carried on cloth or paper, was 

 essentially different to grinding. When followed under the Microscope, 

 easily done when the surface is smeared over with a little aniline dye, no 

 visible pieces of glass appeared to be broken away at all. The polishing 

 began upon the eminences left by the grinding, little facets were produced, 

 and these grew in size, but the polish on the facets appeared perfect from 

 the very beginning. It appeared to the author that the process was a mole- 

 cular one, the upper layer of molecules being operated upon by the polishing 

 material. Not that the fact of not being able to observe structure under 

 the Microscope was any proof that no structure existed till they came to the 

 molecular limit, but the impression obtained from the discontinuity of the 

 two processes, led him to think the material was acted on molecularly. It 

 was an important question which it would be useful to get definitely settled. 

 The author observed that his remarks referred solely to hard materials, and 

 not softer ones such as metals, in the case of which Mr. Beilby, who had 

 investigated the matter, considered that the polishing did not consist only 

 in removing eminences, but also in filling up the pits with the material 

 removed from the eminences. 



After referring to experiments as to the amount of material removed in 

 polishing glass, the author proceeded to discuss observations he had made 

 on the action of very dilute hydrofluoric acid on glass surfaces. The surface, 

 it was found, could be cut away to any required small depth — such as half a 

 wave-length — in a regular manner. By etching two flat surfaces in strips, 

 and continuing these crosswise, the depths could be so chosen as to give the 

 most brilliant colours of Newton's rings. 



The effect of the acid on finely ground glass surfaces, was to eliminate 

 from the roughened surface all the finer irregularities, leaving only those of 

 longer periodicity, and the theoretical reasons for this were explained. 



In the discussion which followed, Mr. Rosenhain adduced reasons for 

 believing that in polishing glass a surface flow, similar to that which 

 Mr. Beilby had shown to occur with metals, did take place. Mr. Horace 

 Beck likewise evidenced facts pointing to an actual transference of glass from 

 the eminences of the surface to the adjacent pits. 



*A Simple Method of Producing Achromatic Interference Bands. 

 By Julius Rheinberg, F.R.M.S. 



This is an account of some experiments on problems connected with 

 microscopic vision which led to an unexpected result in another direction. 

 If a grating is placed on the object-stage of a Microscope and illuminated by 

 a narrow beam of light, it diffracts the light, so that spectra will be formed 



