. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. edouble radiant, and this is fully accomplished in Mr. Brockieslamp there described, while all necessities of centering andfocussing are met by the screw-motion table on which thelamp stands. 105. High-power Demonstrations.—For high-power work,especially biological, I would repeat that a permanent screenor surface of fine plaster of Paris, smooth, and kept carefullywhitened, is of the greatest assistance. The dark parts of theimage are as dark upon it, while the bright parts are muchbrighter
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. edouble radiant, and this is fully accomplished in Mr. Brockieslamp there described, while all necessities of centering andfocussing are met by the screw-motion table on which thelamp stands. 105. High-power Demonstrations.—For high-power work,especially biological, I would repeat that a permanent screenor surface of fine plaster of Paris, smooth, and kept carefullywhitened, is of the greatest assistance. The dark parts of theimage are as dark upon it, while the bright parts are muchbrighter; hence that contrast of shades is heightened whosedilution is one of the great difficulties we encounter. Suchscreens are used in German institutions, and enable resultsto be obtained which otherwise would be beyond the apparatusemployed. Such a surface, and opera-glasses for the back rows,will do a great deal to extend the range of detail possible. In the majority of cases, working with a good projectioneye-piece is decidedly to be preferred. A really fine ^ or -f, THE PROJECTION MICROSCOPE 209. 2IO OPTICAL PROJECTION such as described in § 96, used thus, will give considerablepower, with more depth of focus, and less trouble in finding. The best general distance will be 12 to 15 feet,beyond which the performance of every high-power lensrapidly deteriorates. Even brilliance depends much uponcrispness, and therefore a Eeicherts 7a (\) at 12 feet willgive both more light and better work (though the image is samesize) than his No. 6 Q-) at 17 feet. When the highest poweris needed, it is usually better to use a good oil-immersion lensthan to increase the distance. This refers to the magnificationof minute detail; on the contrary, if the object be to exhibitordinary objects on a colossal scale, this is generally betterdone by a screen distance of not less than 25 feet, withamplifiers or high-power eye-pieces, or often by stretchingthe distance to 50 feet or more. In this way a
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