52 THE MICROSCOPE. 



Mr. Lister proposed to place on the stage of tlie 

 microscope a divided scale, of a certain value ; viewing 

 the scale as a microscopic object, he observed how many 

 of the divisions on the scale attached to the eye-pie.ce 

 corresponded with one or more of a 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 

 ten times less than that 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 employed. 



Mr. Jackson invented the simple and cheap form of 

 micrometer, represented in fig. 34, 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 afford- 

 ing 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 derange. 

 iDent in inexperienced hands. 



The positive eye-piece gives the best view of the 

 micrometer, 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 



to the construction of eye-pieces, the Ramsden is the form by which greater 

 perfection should be obtained. That such an adaptation might be produc- 

 tive of valuable results, appears from Mr. Brooke's statement, that he hai 

 employed as an eye-piece, a triplet objective of one-inch focus, the definition 

 obtained by it being superior to that afforded by the ordinary Hu\ghet,ian 

 eye-piece. An inch or half-inch achromatic object-glass answers extremely 

 well as an eye-piece. 



