410 



On Achromatic Microscopes, with a description of certain 

 Objects for trying their Defining and Penetrating Power, 

 By C.R. Goring, M.D. 



If my readers should be at a loss to comprehend some of the 

 allusions in this paper they will have the goodness to refer 

 to a treatise of mine, in the last number of the old series of 

 this Journal, " On thick Aplanatic Object-glasses for di- 

 verging Rays, &c."* I hope I shall be pardoned if I digress 

 slightly from my present subject to notice the praiseworthy 

 exertions of Mr. Dollond, who has been stimulated by my 

 agitation of the subject of aplanatic object-glasses for mi- 

 croscopes, to apply his vast resources to their construction. 

 His success has been such as might be expected from a 

 descendant of the great father of achromatic lenses. He has, 

 without effort or difficulty, and with the same precision and 

 certainty which he has attained to in the manufacture of the 

 larger glasses, produced three triple small ones, of somewhat 

 less than an inch focus. One is in the possession of the cele- 

 brated Baer ; it has an aperture of about 0-4. When I say 

 that it has given the highest satisfaction to such a judge, 

 every other encomium must be superfluous. The two others 

 I have seen ; they have a naked aperture of 0*45 (for Mr. 

 D. disdains to cut any of it off) : the convexes are both of 

 crown glass. I can scarcely venture to say that they are 

 perfect on opaque objects witK the entierty of their diameter ; 

 but a reduction of 0*05 would, I apprehend, make them so. 

 These glasses are burnished fast into their cells in the most 

 rigid adjustment : this is a great desideratum. Mr. Tul- 

 ley's are only screwed together, and are in consequence de- 

 ranged by the slightest concussion. Mr. D. has combined 

 his two object-glasses nearly in contact. It will not be cre- 

 dited, perhaps, without ocular demonstration, that in spite 

 of the contraction of the focus by this operation, the totality 

 of the aperture of 0*45 is still as good or better than with 



* I had originally intended the present essay to form a continuation of 

 the subject ; but it may, with more propriety, be resolved into a distinct 

 tract. 



