THE MICBOSOOPB. 6\) 



separation into colors at the field-glass is like the over-correc- 

 tion of the object-glass; it leads to a subsequent complete cor- 

 rection. For if the differently colored rays were kept together 

 till they reached the eye-glass, they would then become col- 

 ored, and present colored images to the eye; but fortunately, 

 and most beautifully, the separation effected by the field-glass 

 causes the blue rays to fall so much nearer the centre of the 

 eye-glass, where, owing to the spherical figure, the refractive 

 power is less than at the margin, that the spherical error of the 

 eye-lens constitutes a nearly perfect balance to the chromatic 

 dispersion of the field-lens, and the red and blue rays L' and 

 L" emerge sensibly parallel, presenting, in consequence, the 

 perfect definition of a single point to the eye. The same rea- 

 soning is true of the intermediate colors and of the other pen- 

 cils. 



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

 achromatic object-glass one in which the usual order of dis- 

 persion is so far reversed that the light, after undergoing the 

 singularly beautiful series of changes effected by the eye-piece, 

 shall come uncolored to the eye. We can give no specific rules 

 for producing these results. Close study of the formulae for 

 achromatism given by the celebrated mathematicians we have 

 quoted will do much, but the principles must be brought to the 

 test of repeated experiment. Nor will the experiments be worth 

 anything, unless the curves be most accurately measured and 

 worked, and the lenses centered and adjusted with a degree of 

 precision which, to those who are familiar only with telescopes, 

 will be quite unprecedented. 



The Huyghenean 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. Bamsden, and called, from its purpose, the micrometer 

 eye-piece. When it is stated that we sometimes require to 

 measure portions of animal or vegetable matter a hundred times 

 smaller than any divisions that can be artificially made on any 

 measuring instrument, the advantage of applying the scale to 

 the magnified image will be obvious, as compared with the ap- 

 plication of engraved or mechanical micrometers to the stage 

 of the instrument. 



