38 



CONSTRUCTION OF THE MICROSCOPE. 



sensibly parallel, presenting, in consequence, the perfect definition of a 

 single point to the eye. The same reasoning is true of the intermediate 

 colours and of the other pencils. 



From what has been stated, it is obvious what we mean by an 

 achromatic object-glass : one in which the usual order of dispersion is 

 so far reversed, that the light, after undergoing the singularly beautiful 

 series of changes effected by the eye-piece, shall come uncoloured to 

 the eye. 



The Huyghenian eye-piece, which we have described, is the best 

 for merely optical purposes ; but when it is required to measure the 

 magnified image, we use the eye-piece invented by Mr. Ramsden, and 

 called by him the micrometer eye-piece. The arrangement may be 

 readily understood upon reference to fig. 32. The eye and field glasses 



fig. 32. 



33. 



have now their plane faces turned towards the object ; the rays from 

 the object are made to converge immediately in front of the field-glass ; 

 and here is placed a plane-glass, on which are engraved divisions of 

 1-1 00th of an inch or less. The markings of these divisions come into 

 focus, therefore, at the same time as the image of the object, and both 

 are distinctly seen together. The glass with its divisions is shown in 

 fig. 33, and at it are seen some magnified grains of starch. Thus the 

 measure of the magnified image is given by mere inspection ; and the 

 value of such measures, in reference to the real object, when once ob- 

 tained, is constant for the same object-glass. 



Mr. Lister placed on the stage of his instrument a divided scale, 

 the value of which was known ; and viewing the scale as the micro- 

 scopic object, observed how many of the divisions on the scale attached 

 to the eye-piece corresponded 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 



