Projection Apparatus (1917)

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Bausch & L o m b O ntical Company Path of Light in Projection of Opaque Objects. Difficulties Involved This form of projection is based on the principle of reflection, as indicated by the accompanying diagram, m which A is the illuminant, B condensing lenses, D the opaque object to be pro- jected E the projection lens and F a mirror to direct the image to the screen in correct position. The fewest reflections possible are thus shown to e two, 1 one wishes to produce a picture “ ^reflect,ons mulTLlrm” which at best is a poor reflecting medium. When it is further considered that in the projection of opaque objects the direction o beam of light is altered and considerably dispersed, so that a smaller percen age light rays reaches the projection lens than in the case of lantern slide projectio , is obvious that the screen image so obtained cannot equal in brilliancy that pro- duced from a lantern slide, providing lens equipment and illuminant are the sam . To overcome these handicaps we employ a projection lens of large diameter (4 inches or more) and a more intense hg t. The diameter of the lens in this form of projection is a very important consideration, since, all other things being eqnal, the .Horn,natron *, vary inversely as the square of the diameter of the lens being used. Next in importance to the aperture, or illuminating value, of a lens t quality Of correction for definition and flatness of field, and the larger a given focal length, the more complex become, its correct™. '' J then that lenses used for the projection of opaque objects on our higher priced Balopticons are necessarily of an exceptionally high grade, because of the very a field they give in spite of their large diameter. Means Employed