We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
and the condenser. The shape of the reflector should, however, be ellipsoidal in this case to prevent spherical aberration, which on account of the large angles of incidence near the margin of the reflector may reach enormous amounts. The combination produces one image of the source only when the focal points of the condenser and paraboloid coincide. Efficiency of this construction when used as motion picture condenser exists, probably, not even on paper. The spherical aberration can be partly avoided. Although inspection of Mr. Jenkins diagram shows uneven distribution of light, the efi^ect of the great angle intercepted by the paraboloid is mostly ofl^set by the loss of light on account of the very unfavorable angles under which the light is reflected, especially in the outer parts of the reflector. The centering of such a combination upon a common optical axis is not at all an easy matter, and the lack of centering will cause a one-sided, uneven distribution of light.
The spherical mirror behind the source is undoubtedly a valuable addition. It dates back to the early days of the magic lantern. I used it in 1908, in an apparatus for the projection of compass readings on ships in combination with an incandescent lamp. The filament in these lamps, which were made by the Westinghouse Electric Co., was of grid form and resembled a very steep "M" with rounded corners, over which the image superimposed itself like a *'W", so that no screening of the image by the source took place, except in one point at each bend. The device was patented.
In 1917, the General Electric Co. secured a patent on a modification of this arrangement, limiting it to a parallel coil grid filament, "the area of the spaces between the coils" being equal to the "effective light emitting area of the grid coils," which is the form now so frequently used.
As mentioned above, the gain by the use of the mirror behind the source amounts to 50 to 75 per cent.
The influence of the spherical aberration can be off'set by placing the stencil near the condenser, which is equivalent to a lantern slide arrangement in reduced size. While the condenser could be of the most common kind, it would be necessary to place the source nearer the condenser, than so far has been feasible at least with sources of considerable area. There would be no loss by diaphragm action (II) of the aperture plate. The condenser could, however, not be of the Fresnel type, otherwise the circles separating the zones would be projected into the image as shadows.
A condenser without spherical aberration has the advantage that the stencil may be placed anywhere in the cone, except in a condenser of the Fresnel type, where location near the condenser has to be avoided because of the shadows cast by the rings between the zones.
If the light source is point-shaped, no diaphragm action (II) by the aperture plate will occur wherever the latter may be placed. Diaphragm action (II) by the aperture plate will occur as soon as the light source has an extended area and as soon as the stencil is placed at a distance from the condenser.
A condenser without spherical correction, combined as usual with an extended source, is the least efficient combination, because of the
60