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186 OPTICAL PROJECTION adapted) and require the most precise and delicate adjustment of the object, for which they can afford ample time. The second class consists of biological demonstrators; who may not only need equally precise adjustment, but also to pass parts of a slide in regular succession over the screen, and who likewise have no impatient general audience to grudge the time such manipulation involves. The fitting or short tube—answering to the body of the compound microscope—which carries the objective o, and is screwed with the standard E. M. S. screw, is roughly focussed by the rackwork E 2 , and finally adjusted for higher powers by the fine-adjustment F, substantially made on the pattern so well known in most *histological' stands. This fitting slides into an outer tube, and it is advisable to have several, so that different powers may be ready screwed in place, and simply withdrawn or inserted. 93. Amplifying the Image.—It will often happen that the direct image from the objective alone will not give the best results. Suppose we want to exhibit an object double the size it appears by a ^ lens. We may either double the size of the image shown by that lens, or we may use a lens of double power, \. But the results are different. By amplifying the image we double the whole image, or nearly so, as shown by the ^, and preserve what people roughly understand by its ' depth of focus,' and the larger working distance ; whilst the I lens will show a much smaller portion of the slide, and the light cannot be proportionately condensed into so much smaller a space, though it can be considerably more so condensed. Hence, while really minute and sharp sections or objects are usually shown better by a higher power, more distinct objects of larger size are often best shown with the amplifier, if more power is required. The image is 'amplified' for ordinary objects by inter- posing a plano-concave lens A M between the objective and the screen. This lens fits into the end of a tube, which