Dr. Goring on the new Microscopes, Sec 113 



degree of amplification. Where the power of the achro- 

 matics terminate, we must be content to begin with that of 

 sapphires and diamonds, (precious stones indeed to the 

 optician and naturalist.) As the depth of single lenses is 

 increased, of course their aperture becomes proportionally 

 reduced, and at a certain power gives a cylinder of rays 

 so small that the impression which its light produces 

 on the retina is very faint and diluted ; this causes their 

 aberration, though still as considerable in proportion to 

 their solar foci, as in the largest glasses, to become as weak 

 and insensible (with regard to our visual perception, at least) 

 as their light is ; for the manifestation of aberration, depends 

 on the quantity of rays acted upon, so that a diamond or sap- 

 phire lens of about -^^ inch focus, will show no more aberration 

 than an aplanatic of 0*2 inch, and is equally fit for practical 

 purposes. The new single microscope, therefore, may be said, 

 with certain powers, to have as much superiority over the com- 

 mon glass ones, as the new compounds have over the common 

 rubbish of commerce. Their relation, therefore, to the new 

 refractors and reflectors, also continues about the same as 

 usual, and may be at once comprehended when we consider 

 that the single instruments show the veritable object without 

 aberration, and the compounds an image without aberration. 



I have delivered it as my opinion in the note on Mr. Prit- 

 chard's paper already alluded to, that the new compounds, 

 (when their power is not forced beyond that of ^V of an inch) 

 beat common single microscopes, but of course not achromatics 

 used as single magnifiers; this would be a contradiction. 

 There however will be an hiatus* to be filled up in the 

 single microscope, between the termination of the achromatics 

 at 02 focus, and the beginning of the sapphires and diamonds 

 at about -^-^, for to this depth 1 think they must come, to render 

 their aberration faint enough to cause their performance to 

 compete with that of the aplanatics. The compounds must, 



* This gap may, perhaps, be closed, and the whole range of power rendered equally- 

 perfect, by a set of compound magnifiers of Mr. Herschel's construction, consisting 

 of a meniscus combined with a crossed lens, having the curves calculated for and 

 evecuted in sapphire. This composition executed in glass only, is very superior in 

 performance, and would nearly rival the achromatics, did the chromatic dispersion 

 admit of correction. 



JAN.— MARCH, 1828. . I ' 



