14 MICROSCOPICAL DIAGNOSIS. 



from each other measured with a scale; this distance is divided by 

 the distance between the lines on the stage micrometer, and the 

 result will be the number of diameters. Or, place a scale in front 

 of, and ten inches below the eye-piece; by looking in the instru- 

 ment, keeping both eyes open, the lines of the stage micrometer 

 can be seen resting on the scale, when their distance apart can be 

 measured; divide this distance by the distance between the micro- 

 meter lines, and the number of diameters will be given. 



The magnifying power of a simple microscope is ascertained as 

 follows: The lens is placed on a rest of such a height that its upper 

 surface will be ten inches from the table; a scale is used as a speci- 

 men; one eye being closed -the other is applied to the glass, and the 

 markings on the scale carefully focused; upon opening the closed 

 eye the scale will be seen resting on the table; the distance between 

 the lines as measured on the table is divided by their known dis- 

 tance on the scale, giving the number of diameters magnified. 



Having no eye-piece micrometer, the size of any object is ob- 

 tained in the following way: 4 The image of the specimen to be 

 measured is thrown down upon paper by means of the camera 

 lucida, as were the lines of the micrometer, and carefully measured; 

 this measure is divided by the magnifying power of the microscope, 

 giving the real size of the specimen. Or, the size of the object may 

 be ascertained in the same way the magnifying power was obtained, 

 by holding a rule ten inches below and in front of the eye-piece. 



A microscope is said to be in focus when the relative positions 

 of the object and objective are such that the image is distinct. The 

 following rule should be observed in focusing: Incline the head 

 until one eye is on a level with the stage; with the coarse adjust- 

 ment place the objective near the cover glass, too near to be in 

 focus, then, while looking in the microscope, focus up. Never 

 focus down. If this rule be carefully observed, the breaking of 

 cover glasses and the destruction of specimens will be materially 

 diminished. 



Artificial or natural light may be used for purposes of illumina- 

 tion. For general microscopical work good daylight is to be pre- 

 ferred to any other kind of light. Not the strong sunlight, which 

 is useful only under certain circumstances, but such an even, steady 

 light as can be found by a window looking to the north. 



Nothing can take the place of this northern light, both when 



