Royal Microscopical Society. 1 1 



plain to me that by accident I was getting results closely resem- 

 bling Dr. Pigott's — and this too without the adventitious aid of an 

 aplanatic searcher. I tried to obtain a witness in the person of a 

 lady, as Dr. Pigott did ; but, I am sorry to say, that until I told 

 her to look for " beads," her testimony was not valuable, except as 

 showing either how strong a part the imagination plays in spite of 

 ourselves, or else that I needed an aplanatic searcher to make all clear. 

 As regards this tour de force, I have by dint of a good deal of 

 inquiry and observation obtained a notion which is more or less 

 correct. My notion is that it simply consists of a species of object- 

 glass* of 1 inch or 1^-inch focus (or thereabouts) placed between 

 the eye-piece and observing objective. It is capable (within certain 

 limits)! of being shifted to any distance which practice shows to 

 give the best results. The larger of the two lenses forming the 

 aplanatic searcher is directed towards the observing objective and 

 the smaller towards the eye-piece. Thus explained, some observers 

 may recognize it as a plan which has already been put in practice 

 for amplifying the power in their own microscopes, but discarded as 

 unsatisfactory. Moreover, I have, by the kindness of Mr. Curteis, 

 been favoured with the loan of one of these instruments constructed 

 by him on Dr. Pigott's principles. The " searcher" was a l-|-inch 

 objective originally constructed for photographic purposes by Dall- 

 meyer. 



The results were much better in my hands than I anticipated, 

 and, so long as comparisons (which the proverb says " are odious ") 

 were not made, they might be called giood : but directly this test of 

 their goodness was tried, the verdict went against the " searcher." 

 While there was great gain in amplification there was a palpable 

 loss in definition : less certainly than I was prepared to expect, but 

 still a palpable loss. The objectives used in this examination were, 



i Smith and Beck, 



^ Powell and Lealand, 



i Andrew Koss, 



1^ Powell and Lealand (dry), 

 -rg^ Merz (immersion), 



and each and all of them performed better without than with it, so 

 far as giving a brilliant and well-defined image. The objects chosen 

 were Fleurosigma Formosum (dry), and the whole series of Thysa- 

 nura scales. Mr. Fitzgerald assisted me in the examination, and I 

 beheve came to the same conclusion as myself. 



On a second occasion I used an ordinary 1^-inch objective by 

 Eoss as a " searcher," t and obtained such good results that I was 

 nearly altering my judgment upon it. But, remembering that I 



* Does it differ from an ordinary object-glass more than object-glasses of the 

 same focus by different makers differ from each other? 



t The contrivance by which this is effected is ingenious. 



j Subsequently I tried a |-inch by Smith and Beck with good results. 



