MICKOSCOPIC APPARATUS. "771 



light are represented as proceeding from the centre of the object, and 

 three from each end of it. These rays, if iminterrupted, would form 

 an image of the object at a, but owing to the interposition of the 

 field-glass c, they are refracted so as to converge and meet at 6, 

 where the diaphragm is placed to intercept aU light except what is 

 necessary for the formation of a perfect image. The image formed at 

 6 is viewed as an original object by the observer through' the eye- 

 glass e. 



MicEOSCOPic Apparatus. — ^In measuring the size of microscopic 

 objects, a micrometer (fiixgog, small, and fi'eTgov, a measure) is em- 

 ployed. The stage micrometer consists of a piece of glass, ruled with 

 fine lines by means of a diamond point, at some known distance apart, 

 such as the Troth, or xjni-irth, or TrAirth of an inch. A mode of ascer- 

 taining the magnifying power of the compound microscope is founded 

 on the assumption that the naked eye sees most clearly and distinctly 

 at the distance of ten inches. If a divided scale be placed on the 

 stage, and distinctly seen magnified through the instrument, let a rule 

 be held at ten inches' distance from the right eye, while the observer 

 uses, at the same time, his left eye in looking at the other scale 

 through the microscope, and let the rule be gently moved so that it is 

 seen to overlap or lie by the side of the magnified picture of the other 

 scale, — a comparison as to how many of its known divisions corre- 

 spond with a number of those on the magnified scale wiU indicate 

 the magnifying power. Upon a similar principle a pair of compasses 

 may be substituted, whose points being placed on the stage are sepa- 

 rated till they cover or mark off so many spaces as magnified by the 

 instrument. If they cover one magnified space, and correspond to 2, 

 3, or more, known spaces on the rule, then the instrument is said to 

 magnify 2, 3, or more times linear that known space. If xiirth of an 

 inch is found to cover 2 inches on the rule, the instrument magnifies 

 200 times ; if 3 inches, 300 times ; if 4 inches, 400 times, and so on. 

 In this way is determined the magnifying power of any combination 

 of lenses, and the scale which is magnified is called a stage-micrometer. 

 The size of objects may be measured by placing them directly on this 

 micrometer ; but it is obvious that they cannot under high powers be 

 brought into focus at the same time as the lines of the micrometer. 

 An instrument called the eye-piece micrometer is therefore generally 

 used. It consists of a fine scale, ruled on glass, and placed in the 

 focus of the upper glass of the eye-piece. The value of each space of 

 the eye-piece micrometer varies with the magnifying power of the 

 object-glass which is placed on the microscope ; e.y., suppose we look 

 at TTsVirth inch space of the stage micrometer with a magnifying power 

 of 250 diameters, and find that the space thus magnified extends over 

 5 spaces of the eye-piece micrometer, the value of each space of the 

 latter will obviously be 3T!Virth inch when a power of 250 diameters is 



