Page:The American Cyclopædia (1879) Volume XI.djvu/538

 520 MICROSCOPE divisions of the eye-piece micrometer. The French scale is the millimetre divided into 100 or 200 parts. The eye-piece micrometer, known as Jackson's micrometer, is a finely ruled glass scale, introduced by means of an opening be- tween the field and eye lens of the eye piece, so as to be in the focus of the eye lens. The value of the scale is determined by placing the stage micrometer on the stage, and viewing the divisions with the given objective and eye piece; thus, if 10 divisions of the eye microm- eter corresponded with one (-01") of the stage micrometer, then the value of one division of the eye-piece micrometer would be "001" with that particular object glass and eye piece. The stage micrometer being replaced by any object, its dimensions may be readily ascertained by noticing how many divisions of the scale are subtended by it. The micrometers of this de- scription are very convenient, and, when care- fully used, accurate. The definition is slightly injured, however; this objection is obviated by Mr. Tolles's solid micrometer eye piece. If the observer has only a stage micrometer, the divisions may be projected on paper by means of the camera lucida ; then, with the same ob- jective and eye piece, the image of any object being projected on the paper, its dimensions are at once ascertained. Messrs. Powell and Lealand furnish a cobweb micrometer suscepti- ble of great accuracy; it is similar to that used for astronomical purposes. Messrs. Grunow have somewhat improved upon Powell and Lealand, and their cobweb micrometer with orthoscopic eye piece is very delicate. They also furnish Fraunhofer's stage micrometer, which has the advantage of giving the absolute dimensions of the object, without reference to the power of the objective or eye piece. With careful use the eye-piece micrometer is as ac- curate as any of these, and much less expen- sive. The polarizing apparatus consists of two Nicols prisms of calc spar with revolving fit- tings, one (called the polarizer) designed to be placed below the object, the other (termed the analyzer) above, either directly over the eye piece, where it generally cuts off part of the field, or at the lower end of the draw-tube; here, if the prism be good and not too long, it will not much affect the definition, and will allow the whole field to be visible. Very much depends upon these prisms. Those supplied by the Messrs. Grunow are very large and fine, and the analyzer is placed by them directly above the objective ; it is too large, however, to be placed so low down without injury to the definition. Beck's analyzer is much shorter than Grunow's, and may be used either over the eye piece or placed in the end of the draw- tube. There is much difference in these prisms ; some scarcely injure the definition at all, and others are very poor. The polarizer is usually somewhat larger than the analyzer. The calc spar is very soft, and, if not protected by thin glass covers, liable to injury. A set of revolv- ing selenites, to go below the object, between it and the polarizer, accompanies the prisms. There is not much choice in the method of mounting and revolving these prisms ; perhaps Grunow's is the most elaborate, but, when the revolving selenites are included, we think Beck's arrangement the most complete. A " selenite stage " is often employed, simply placed under the object, on the stage of the microscope. Mr. Darker has contrived a stage of this kind, in which the selenites revolve. A complete po- larizing apparatus is very important, and should be attached to every good instrument. Some- times tourmalines are used instead of the prisms of calc spar; they are objectionable on account of their color, but placed over the eye piece do not at all obstruct the field of view. The camera lucida furnishes the means of drawing or sketching outlines of objects viewed in the microscope, some provision for which is abso- lutely necessary. Often this is merely a plate of neutral tint glass, which, placed in front of the eye piece, at an angle of 45, when the microscope is turned horizontal, reflects the image to the eye, and at the same time pencil and paper upon the table may be viewed through the "glass. A better contrivance than this is the steel disk of Sommering, made slightly smaller than the pupil of the eye ; this, when placed in front of the eye piece, enables one to view object and paper at the same time. In these contrivances, and also in the Nachet drawing prism, the object, being viewed after but one reflection, is reversed right and left. Where it is necessary to finish a drawing by the eye, this is a serious diffi- culty ; it is therefore preferable in all cases to use the Wollaston prism, which is applied so as to give an unobstructed view of the whole field, and with which the drawing is precisely as it appears in the microscope without the prism. A little practice is required to use it well, but if the observer will take care to have the paper strongly illuminated, while the ob- ject is only enough illuminated to be seen dis- tinctly, no great difficulty will be found. But whoever does much original work with the microscope should not depend on drawings made with the camera lucida, but should learn to photograph the objects he has discovered, or the histological preparations he has made. The eminently successful work of Drs. Edward Cur- tis and J. J. Woodward, at the army medical museum in Washington, should encourage all who have the means to adopt this accurate method of permanently securing the results of their labors. The animalcule cage is a simple contrivance by means of which a drop of water may be retained between two glass plates, which may be approximated so as to jnst con- fine the object, without allowing, if it be liv- ing, too much freedom of movement; it is often made to serve the purpose of a com- pressor, for crushing soft bodies more or less during the examination. The stage forceps is exceedingly useful for the examination of small insects. At one end is usually placed a bit of