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458 THE PHOTOGRAPHIC NEWS. [July 20, 1883. harmless drollery, Mr. Edouin captivating his audience with a thousand quaint doings and sayings. There may be some trifles of inaccuracy about the play, such as lighting the model from behind, instead of in front, and speaking of the ferrotype process forty-five years ago, when photography, let alone tintypes, was not invented; but these are details we must leave to the carping critic to find fault with. . Patent Entelligence. Application for Letters Patent. 3476. RICHARD Brown, ROBERT William Barnes, and Joseph Bell, all of Liverpool, in the county of Lancaster, for an in vention of “Improvements in means for and method of pro ducing designs upon paper or other fibrous or soft material, or upon metallic surfaces.”—Dated 14th July, 1883. Specification Published during the Week. 5110. A. M. Clark, “ Producing designs, &c., upon glass,metal, and other surfaces.”—A communication from H. Beau. This invention consists in a novel process of producing or applying designs, letters, figures, or other marks upon glass, metal, or other surface, in such manner that the said surface forms a ground upon which the said designs, letters, figures, or other marks appear to be printed or engraved directly. The process consists in printing, or otherwise producing the designs, letters, figures, or other marks, upon an absolutely colourless and transparent tissue or film, which is afterwards applied and caused to intimately adhere to the glass, metal, or other surface. The tissue or film when so applied becomes completely invisible, the natural colour and aspect of the subjacent surface showing through it unaltered, leaving the designs, letters, figures, or other marks, alone visible on the transparent or other ground with which they appear to be intimately incorporated. The tissue or film which covers the entire subjacent surface also serves to protect it from atmospheric influences. In carrying out this invention, I print or otherwise produce the desired designs, letters, figures, or other marks, upon thin unsized bank post paper or other similar material, such as china paper, china crape, and thin fabrics of jute, ramie fibre, and generally fabrics permeable to varnish. After printing, the tissue is subjected to the application of a colourless varnish,’and stove dried. By this operation the substance of the tissue or fabric is caused to disappear from the eye, there remaining only a very thin flexible colourless pellicle or film bearing the impression upon it in any desired colours. By printing with suitable “ reserves,” it will be understood that transparent colourless designs, letters, figures, or other marks, may be produced upon a coloured transparent ground if desired, and by conbining the various results producible by the aid of lithography, colours, and drawing, all kinds of effects may be obtained. By printing, for example, in blank upon a thin permeable pellicle or film as above described, it will, when applied to glass, produce an imitation of engraving with hydro fluoric acid as usually produced.—Provisional Protection. Patent Granted "in United States. 279,878. Edward J. Muybridge, of San Francisco, Cal., for “ A method of and apparatus for photographing changing or moving objects.”—Application filed 31st August, 1881. Renewed 19th April, 1883. No model. LESSONS IN OPTICS FOR PHOTOGRAPHERS. BY CAPTAIN W. DE W. ABNEY, R.E., F.R.S. Lesson IL—Refraction of Light. When light enters from one medium into another, it is a matter of common experience that two things (such as from air and water) happen ; first, part of it is bent from its original direction, except in the case when it strikes the surface bounding the two media perpendicularly ; and second, part is reflected. This latter obeys the laws we have already mentioned; but in what way is the ray of light bent? The law or rule which the bending follows has been found by experiment. Supposing a ray of mono-chromatic light (a light of one colour), A O, strike a surface (say) of water at O, instead of continuing its course to a, it is bent to A', and another ray of light, B O, striking at a different inclination, instead of continuing its course to b, is bent to B'. Describe any circle with O as a centre, and draw P O Q perpendicular to d O d, the surface of the water, and b m, a n, B‘ 0, A' P, perpendicular to P Q. It is found by experiment that n a bears the same proportion to A' P that b m does to B' O. In the case of water the proportion is about as 14,to1. For light flint glass, the proportion is about 14 to 1— that is, a n would always be 1} times larger than A' P, and b m 1} times larger than B O. This proportion is called the index of refraction, and is expressed mathematically by saying that the sine of angle of inclination bears a con stant proportion to sine of the angle of refraction. I® England this proportion is usually designated by the Greek letter u. We have above taken light as going from a rare medium to one more dense ; but if it goes from the dense medium (such as glass) to a rarer one (such as air), the ray, instead of being bent towards the line which is perpendicular to the common surface of the two (P Q in fig. 7), would be bent away from it, and a proportion would still hold good as before described ; but such proportion would be exactly the reverse to that which would be found above. Thus when a ray travels from air to glass, the proportion of the perpendiculars an to A' P was 14 to 1; but if the ray travelled from glass to air, it would be 1 to 14. It will now be seen that if the course of a ray of light passing through a plate of glass in air with parallel side’ Fig. 8. be traced on the same principles as adopted in fig. 7, 1C will always emerge parallel to its original direction. The student’s attention is here directed to an importants deduction. Suppose we have a ray of light which traversing a dense medium, and tries to get out into.nt less dense (from glass to air, suppose), it may happen th"