S MANUAL OF HISTOLOGY. 



of divisions of the wooden rule corresponding to a single divi- 

 sion of the micrometer slide as seen with the other eye directed 

 through the tube of the microscope. 



By this method of double vision, as it were, a comparison is 

 instituted between the two rules, and the ratio that one bears 

 to the other may be estimated. 



Suppose, for example, that T <rVo of an inch on the scale of the 

 stage micrometer is equal to W of an inch on the wooden rule. 

 The ratio of T^W to & will represent the magnifying power 

 of that particular combination. Reducing these fractions to 

 a common denominator they stand to one another as 1 to 200. 

 The object has therefore been magnified two hundred times. 



With a short eye-piece the power is greater and it increases 

 in proportion as the tube is drawn out. It is customary how 

 ever to assume a certain length of the draw-tube as the stand- 

 ard : this is twenty-five centimetres or about eight inches. 



How to estimate the size of an object. To estimate the size 

 of an object is a much easier task. Place the stage micrometer 

 upon the stage of the microscope and then slip the micrometer 

 eye-piece into the draw-tube. The micrometer eye-piece is 

 simply an ordinary ocular with a glass cover fitted into the 

 diaphragm. The micrometer consists of a series of parallel 

 lines ruled across it at regular distances apart. By focussing 

 the lines on the stage micrometer one may readily count the 

 actual fractions of an inch corresponding to a single division 

 in the micrometer eye-piece. 



Thus, for example, if we find that a single division of the 

 micrometer eye-piece corresponds to T5 Vir of an inch, and that 

 a lymphoid corpuscle covers half a division, its diameter is 

 necessarily -g-^ of an inch. 



Testing a lens. A lens should be free from certain defects, 

 as we have already stated. First of all it should have no 

 spherical aberration ; the objects seen upon the edge of the 

 field should be sharply defined, and all objects having parallel 

 sides should appear as such. In other words, they should not 

 be distorted. 



Secondly, they should have no color or, at least, as little 

 as possible. This defect, however, has never been entirely 

 overcome ; some glasses are over-corrected and then the pre- 

 vailing color is blue ; others are under-corrected and then the 

 prevailing color is red. 



