CONSTRUCTION OF THE MICROSCOPE. 51 



object-glasses answer extremely well for this purpose ; and 

 as the sets are usually made removable, the front pair can 

 be readily separated for the experiment. 



Mr. Lister places on the stage of his microscope a 

 divided scale, the value of which is known ; and viewing 

 the scale as the microscopic object, observes how many of 

 the divisions on the scale attached to the eye-piece corre-' 

 spond with One of those in the magnified image. If, for 

 instance, ten of those in the eye-piece correspond with one 

 of those in the image, and if the divisions are known to be 

 equal, then the image is ten times larger than the object, 

 and the dimensions of the object are ten times less than 

 indicated by the micrometer. If the divisions on the 

 micrometer and on the magnified scale are not equal, it 

 becomes a mere rule-of-three sum ; but in general this 

 trouble is taken by the maker of the instrument, who 

 furnishes a table showing the value of each division of the 

 micrometer for every object-glass with which it may be 

 used. 



Mr. Jackson invented the simple and cheap form of 

 micrometer, represented in fig. 35, which he described 

 in the Microscopical Society's Transactions, 1840. It 

 consists of a slip of glass placed in the focus of the eye- 

 glass, with the divisions sufficiently fine to have the value 

 of the ten-thousandth of an inch with the quarter-inch 

 object-glass, and the twenty-thousandth with the eighth ; 

 at the same time the half, or even the quarter of a 

 division may be estimated, thus affording the means of 

 attaining all the accuracy that is really available. It 

 may therefore entirely supersede the more complicated 

 and expensive screw-micrometer, being much handier 

 to use, and not liable to derangement in inexperienced 

 hands. 



The positive eye-piece gives the best view of the micro- 

 meter, the negative of the object. The former is quite 

 free from distortion, even to the edges of the field ; but 

 the object is slightly coloured. The latter is free from 

 colour, but is slightly distorted at the edges. In the 

 centre of the field, however, to the extent of half its 

 diameter, there is no perceptible distortion ; and the 

 clearness of the definition gives a precision to the measure- 



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