Dr. Goriug on Achromatic Microscopes, 413 



because he thought that, as in the Gregorian construction, 

 the rays must necessarily pass over a considerable space from 

 the small metal before they reach the eye-piece : the slightest 

 errors would become wonderfully aggravated in this long 

 compound-reflecting microscope (for such it is in its operation 

 on the image of the large speculum). In Sir I. Newton's 

 construction the eye-piece is as close as possible to the small 

 metal ; and it is this circumstance which, in my opinion, con- 

 stitutes the superiority of the Newtonian over the Gregorian 

 construction, which is, I believe, pretty generally admitted. 



Now, this analogy will apply to achromatic microscopes, 

 though the errors of rays by refraction are very small in pro- 

 portion to those by reflection ; but on theoretical consi- 

 derations I am not disposed to lay much weight, unless they 

 are confirmed by practice. 



Having thus stated the pour and contre of the question, 

 I leave every man to the exercise of his own private judg- 

 ment. I am not such a despot as to expect people to use 

 long or short tubes (when either will do very well) just to 

 chime in with my fancies on the subject, any more than I 

 should wish them to break their eggs at the large or small end, 

 according to my own ideas of convenience. The case seems 

 to me to resolve itself into a matter of opinion. The only 

 real advantage I can discover in a long tube, is to procure a 

 very extensive scale with a shallow eye-piece for microme- 

 trical measurements, which it certainly accomplishes. 



Eye-pieces. — Those used with the aplanatic object-glasses 

 are of the negative Huygenian kind, very similar to those 

 employed in telescopes, consisting of two plane convex 

 lenses only. It is well known that these can be rendered 

 perfectly achromatic ; and their spherical aberration being 

 incorporated with that which is corrected by the concave lens 

 of the object-glass, the whole of the instrument is perfect 

 though the constituent parts are not. It is not thought advise- 

 able to extend the number of glasses beyond two, for the same 

 reasons which forbid the use of a third or fourth glass in 

 telescopes. If, however, people are to be found so besotted 

 with large fields of view as to prefer them to all the valuable 



