170 PRACTICAL TREATISE ON THE MICROSCOPE. 



coloured images to the eye ; but, fortunately, and most beauti- 

 fully, 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 consti- 

 tutes 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 defi- 

 nition 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 that 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 uncoloxu"ed to the eye. We can give no 

 specific rules for producing these results. Close study of the 

 formulae for achromatism, given by the celebrated mathema- 

 ticians we have quoted, wiU do much, but the principles must 

 be brought to the test of repeated experiment. Nor will the 

 experiments be worth anything, unless the ciu-ves be most 

 accurately measured and worked, and the lenses centred and 

 adjusted with a degree of precision which, to those who are 

 familiar only with telescopes, will be quite unprecedented. 

 When object-glasses of high power, constructed upon the 

 improved plan of Mr. Lister, came to be accurately tested, 

 Mr. Koss found out that if they exhibited well, objects that 

 were uncovered, they did not show so beautifully those that 

 were covered with thin glass or mica, with or without being 

 immersed in fluid. He speedily found a remedy, which, in 

 order to be fully understood, must be described iu his own 

 words, taken either from the original paper in the Transac- 

 tions of the Society of Arts, or from the article, " Microscope," 

 in the Penny Cyclopaedia, which has been already so often 

 quoted. 



" Mr. Lister's new principles were applied and exhibited 

 by Mr. Hugh Powell and Mr. Andrew Koss, with a degree 

 of success that had never been anticipated ; so perfect, iadeed, 

 j^ere the corrections given to the achromatic object-glass — 



