Page:The American Cyclopædia (1879) Volume XI.djvu/536

 518 MICROSCOPE in their stands is the attachment of the slow movement at the lower part of the tube of the microscope itself. In this respect the stands of Ross, Powell and Lealand, Spencer, Tolles, and the Messrs. Grunow are superior. With the high powers, the springing of the tube when the finger is applied, and the shaking when the adjustment for cover is attempted, are disagreeable. The larger students' micro- scope made by Beck is a very fine instrument ; the stage movements and adjustments are very complete. The stands furnished by Ross are heavy and cumbersome; they are, however, exceedingly steady, and finished with extreme care. Those supplied by Powell and Lealand are much lighter, and have some peculiar ad- vantages. The accessories supplied by this firm are more complete than those of any other ; the stage of their new and largest in- strument is very thin, and allows greater ob- liquity of illumination than that of Beck. Their achromatic condenser is beautifully fin- ished; it has an angle of 170. The stand itself is not as steady as that of Beck, but the fine adjustment is more conveniently placed. The stands furnished by Messrs. Grunow are of different patterns, but all excellent in beauty of finish, smoothness of adjustment, and steadi- ness, comparing most favorably with the Eng- lish work. The stands furnished by Spencer and Tolles have many excellences, and are very steady. All good instruments should have a graduated draw-tube within the main tube, and the latter should be not less than 1-4 inch in diameter. The draw-tube is absolutely necessary for micrometry, and is very conve- nient to receive the analyzing prism, erector, or Tolles's amplifier. Two adjustments for focus are also necessary, one quick by rack- work and pinion, the other very delicate by screw. The pinion heads should be large, to allow of most delicate movement, and Messrs. Beck and Zentmayer graduate the head of the screw of the fine adjustment, so that the thick- ness of covering glass may be measured. A skilful observer may dispense with the rack movement, and produce the approximate ad- justment by sliding the tube. This is the method adopted in most of the French instru- ments, and in the " educational " of Beck. The stage movements should be smooth, and but slightly disturb the adjustment when in focus. The adjustment for wear in the Eng- lish instruments is by spring, in the American by screws; the former involves more work for the maker, but is better. The lever stage, though performing finely when first from the hands of the maker, is much more liable to derangement than that in which the motions are produced by rack and screw. In the Beck instruments the rack and screw are both so low that the latter passes entirely under the bottom of the stage, which is consequently much thinner than it can be made when the screw is introduced between the movable plates; the milled heads themselves are thus dropped below the level of the stage, and this is deemed of great importance ; this is also the case with Powell and Lealand's large micro- scope. One of the most important of recent improvements in the microscope has been the adaptation of the instrument to stereoscopic binocular vision. The binocular microscope, as we now have it, is the result of gradual progress in the application of Prof. "Wheat- stone's discovery of the stereoscope, from the year 1851, when Prof. Riddell of New Orleans first attempted the production of micro-ste- reoscopic relief, to the recent simple and uni- versally used invention of Mr. Wenham. M. Nachet, Prof. H. L. Smith, Messrs. Powell and Lealand, Mr. Tolles, and Dr. Barnard have also devised special plans to obtain the same effects. Some of them have their own special advan- tages, but Mr. Wenham's recent invention is so simple that it has been almost universally adopted by American and English makers. We will therefore only describe here his in- vention, and refer the reader who desires in- formation of the plans of the above named gentleman to Beale's "How to Work with the Microscope" (London, 1868) ; to Carpenter " On the Microscope " (London, 1868) ; and to the report of Dr. F. A. P. Barnard " On Ma- chinery and Processes of the Industrial Arts, and Apparatus of Precision," exhibited at the Paris exposition of 1867. The pencil of rays which has passed through the objective I n is cut in half by the interposition of a tra- pezoidal prism, as shown in fig. 3, A ; half of the rays of the pencil pass up the tube as usual, and the other half enter the anterior face of the prism and are reflected from the lateral surfaces 5 and c, and emerge at d to