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James G. Pennycuick
5 of 12
SPECIFICATION forming part of Letter Patent No. 579,350, dated March 23, 1897.
Application filed May 4, 1896. Serial No. 590,131. (No model.)
To all whom it may concern:
    Be it known that I, JAMES G. PENNYCUICK, a subject of the Queen of Great Britain, residing at Toronto, in the county of York and Province of Ontario, Canada, have invented a certain new and useful Improvement in Shades or Globes, of which the following is a specification, reference being had to the accompanying drawings.
    This invention is designed to provide a shade or globe particularly adapted for arc-lights, but applicable also to other purposes where the diffusion of light is desired.
    It is well known that the intense light given out by the electric arc requires to be softened to avoid injury to sight. This is generally done by the aid of frosted or ground glass globes. These, however, obscure the light to such a degree that probably not half of the light given off by the arc is utilized when a ground-glass globe is used. To overcome this objection is the object of this invention, which is accomplished by the construction hereinafter more particularly described and then definitely pointed out in the claims.
    In the accompanying drawings, Figure 1 is a side elevation of a shade constructed according to my invention with part broken away. Fig. 2 is a cross-section of the same. Fig. 3 is a cross-section of a similar shade having its body in two parts. Fig. 4 is an enlarged sectional detail. Figs. 5, 6, 7, and 8 are outline views, on a smaller scale, of different forms of shades or globes on which the principle of my invention may be carried out, the prismatic ribs of the curved sides of said figures being shown in the enlarged sectional detail above described as Fig. 4.
    Referring now to the details of the drawings by figures, 1 represents the body of the shade, which is preferably octagonal in cross-section, as clearly shown in Fig. 2, and having vertical sides 2 and inclined bottom 3. On each side or panel are ribs or prisms 4, the lines of which are made of such a shape that the rays of light from the arc striking the inside of the upper ribs will be totally reflected in a downward direction, as indicated in Fig. 1, while light striking the lower prismatic ribs will be refracted and bent from
its downward course slightly toward the horizontal.
    I show a reflector or cover 5 at the top, but this may be left off or changed in form without departing from the spirit of my invention. One standing rule should, however, be observed in shaping the mold for making the prismatic ribs, and that is that the angles or upper edges of the prismatic ribs should vary only through a downward angle of ninety degrees from a perpendicular to the vertical axis of the shade and that the angles of the lower edges of the prismatic ribs vary only through an upward angle of ninety degrees from the same perpendicular. If this rule is not observed, it is rather difficult to mold a shade with prismatic ribs, as some of their faces would form dovetails, which would prevent the parts of the mold of the ordinary construction being withdrawn sidewise from the finished article. As shown in the drawings, the upper face of each prism or prismatic rib makes substantially an angle of one hundred and five degrees to the surface of the side of the shade, while the lower face similarly makes an angle of forty-five degrees.
    Although I show and describe a shade as being molded of a polygonal shape, yet a measure of utility may be obtained with a shade round in cross-section. The diffusive power of the prismatic ribs is much increased, however, by the use of the polygonal form, as there is a tendency to focus and so concentrate the light when the shade is made globular or circular in cross-section, which is avoided when the shade is made of polygonal form. This I have proved by experience, as I found that with the ribs running around the globe or shade in curved lines the light from the lamp appears to be focused to a considerable extent in a vertical line directly between the light and the spectator, but with the shade having a polygonal shape, as the ribs are flat or substantially so, this "focusing" is avoided and the light is diffused over the entire surface of the globe or shade.
    I have described so far only one style of shade, but do not limit myself to this form, as many different kinds of shades may be made without departing from the spirit of my invention,