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Bausch & Lomb Optical Company The projection microscope was quite widely used throughout Europe by the middle of the 18th century, although, of course, in a very crude form. Both sun- light and lamplight were used as illuminants, and the optical and mechanical devices were of a primitive nature. The scientific endeavor which finally led to the present high Efficiency development of this instrument was concerned with the improve- ment of illuminating devices, as well as optics and mechanical appliances for operating the instrument. The compound projection microscopes, which we offer to-day, are as high grade scientific instruments in every respect as any regular table microscope and provide possibilities not to be had in regular microscopes. While some may have experienced a feeling of doubt in the past as to the real efficiency of microscopical projection, they may now rest assured of obtaining absolute satisfaction with any of our higher grade outfits, especially designed for this kind of work. One of the latest developments in microscopical projection we offer in our combined horizontal and vertical type of microscope, which provides for the projection of microscopic objects held in a horizontal plane as well as in the usual manner. The small diameter of the microscope objectives and the high Problem o magnifications which they produce require intense and uniform Illumination 6 . „ illumination of the slide or object to be projected. 1 o accomplish this we have adapted the scheme originally introduced by Kohler, which consists of projecting the evenly illuminated image of a condensing lens into the object slide by means of a substage condenser. i V * u Figure I—Kohler Illuminating System. The arrangement is shown in the accompanying diagram. It consists of a light source, L, a single condensing lens of suitable focus, C, a diaphragm, D, a substage condenser, E, with an object slide, S. The condenser, C, forms an in- verted image of the light source, L, at L', or the plane of the iris diaphragm of the substage condenser, E. The substage condenser is so placed that it focuses the image of the first condenser, C, in the plane of the object, giving C'. The size of the illuminated aperture is regulated by the diaphragm, D, of the condenser, which diaphragm is imaged by the substage condenser in the plane of the object slide at a point nearly coincident with C\ giving a sharply defined margin of the projected image. The Kohler illuminating system is made use of, without modification, in our Simplified Micro-Projection Apparatus. (See page 83 ). 10