772 MICROSCOPIC APPAKATUS. 



used. If a lower object-glass were taken — e. g., one which magnifies 

 50 diameters, it would then be found that the nrSTjth inch space of 

 the stage micrometer is equal to only one space of the eye-piece 

 micrometer, so that, with this magnifying power, each space of the 

 latter indicates Timrth inch. These calculations have to be made for 

 the magnifying powers of every microscope. When an object is to be 

 measured, the stage micrometer is removed, and the object, placed on 

 a slide and covered in the usual manner, is brought iato focus, say, 

 with a power of 250 diameters. If the object extends over 5 spaces 

 of the eye-piece micrometer, its breadth would evidently be nmr inch. 

 In using the eye-piece micrometer, the marked side of the glass is 

 put undermost. Hartnack's eye-piece micrometer is the best. With 

 this instrument, when using -a magnifying power of 500 or 600 

 diameters, we can estimate distances from ■oVifth to TTrsth of an inch 

 with tolerable precision. Other kinds of eye-piece micrometers are 

 also employed, such as the cobweb micrometer, where, by the motion 

 of a delicate screw, fine wires, extended across the field of vision, can 

 be separated from each other to known distances. 



In delineating minute structures, it is useful to have the image 

 thrown on paper by means of a camera-lucida, or small prism, which 

 can be easily attached to the microscope. The microscopist sometimes 

 uses a eompressorimn, for the purpose of applying pressure to objects 

 whilst they are under examination ; troughs for holding such plants as 

 Ohara, which are to be seen in water ; whUe various instruments for 

 the dissection and examination both of animal and vegetable struc- 

 tures are indispensable accessories. In testing the power of an in- 

 strument, certain objects are used, in which peculiar markings occur, 

 which can only be properly seen by a fine instrument. Either artifi- 

 cial or natural objects may be chosen as test-objects. The former have 

 been prepared by Nobert, a Konigsberg optician, and consist of glass 

 plates, on which are ruled, with a diamond, systems of a hundred lines, 

 which, 10 by 10, approach closer together and are finer, according to 

 a definite standard. With most instruments only the 6th and 7th 

 systems can be distinctly made out to be composed of separate Hues. 

 Superior instruments reach the 8th and 9th. No instrument has yet 

 reached the 10th system, with its component lines. The best test- 

 objects are the natural ones, as being regular and uniform in their 

 markings, such as the scales of Podura plumbea or common Spring- 

 taU, of Lepisma saccharina or sugar-louse, and the minute markings 

 of the Diatomacese, as Pleurosigma Hippocampus. Certain markings 

 occur in these test-objects, which can only be seen properly by good 

 microscopes. 



MicEoscopic Maniptjlation. — In viewing objects under the 

 microscope they must be placed on slips or slides of glass, which 

 should be of a uniform size, not less than three inches by one ; and 



