110 JOURNAL OF THE [April, 



But now how perverse and prejudiced all that opposition 

 seems, and how simple and reasonable the new system of 

 numerical aperture is seen to be ! 



Before our time, the fight was fought over the binocular body, 

 the achromatic objective, and even the "compound" principle 

 itself. I happened, only the other day, to take up one of Dr. 

 Hill's volumes published in 1752,^ in which I found an illustra- 

 tion of the spirit of opposition of which I have been speaking ; 

 for he iterates and reiterates his belief in the general superiority 

 of the single microscope over the compound, for all genuine 

 investigation. We can easily believe that the compound micro- 

 scope, previous to the invention of the achromatic objective, was 

 a rather inefficient instrument. Still, in the light of our present 

 knowledge, it is exceedingly amusing to read such a passage as 

 the following : 



" Thus much I have thought it necessary to say in Favour of 

 the Use of single Magnifiers of great Power, in the more nice 

 Investigations, because I know their being difficult and Dis- 

 agreeable in the using has thrown them into an unmerited dis- 

 regard, a Neglect that Will clip the Wings of all succeeding 

 Discoveries. The Microscope, as we hear of it in the Hands of 

 Lewenhoek, and in those of all the other Authors, who have so 

 amazingly seen the Minima of Nature, and who have inspired 

 the World with a Love for its Investigations, was a single Glass of 

 this kind. Almost all the great Discoveries which have rendered 

 the Instrument famous, were made by single Glasses. These 

 are the only ones to trace with Accuracy the Ways of Nature in 

 these her minutest Productions ; nor are those who are ac- 

 quainted only with the Use of that Plaything the double Micro- 

 scope, to wonder that they cannot follow the Discoveries of Men 

 who have used these single Glasses in the making them ; or 

 accuse People of Imposition or Fancy who have used in their 

 Investigations an Apparatus which is so superior in real Value to 

 that by which they vainly attempt to follow their steps. The 

 double Microscope is the Instrument for those who would be di- 

 verted by the Powers of magnifying, but this is that which ought to 

 be understood and employed by all who would make real Discov- 



1. Essays in Natural History and Philosophy, containing a Series of Discoveries 

 by the Assistance of Microscopes, by John Hill, M. D. 



