THE 



MONTHLY MICROSCOPICAL JOURNAL. 



MAKCH 1, 1871. 



I.— ANNUAL ADDKESS * 



In surveying the work of last session the Society may find much 

 reason for congi-atulation, from the variety and importance of the 

 papers brought before it. In a remarkable series of contributions 

 Dr. Pigott has made us acquainted with the nature and extent of 

 uncorrected aberrations in our best object-glasses. He has explained 

 more fully than had been done before, the advantages to be gained 

 by the immersion system; he has called attention to sources of 

 fallacy arising from what he terms ei8(o\a (eidola), found above and 

 below the true focus ; he has devised new tests, and introduced a 

 new mode of correcting aberration by the use of his "Aplanatic 

 Searcher." The novelty of this invention consists in searching the 

 axis for best focal points at places intermediate between a fixed eye- 

 piece and the objective, and applying new and variable corrections 

 there. The aplanatic searcher also permits the use of a low eye- 

 piece of 3 inches focus (the usual A being 2^). Fm^ther than this, 

 it is said to increase focal depth, raise the power of the objective, 

 improve the definition of parts seen in perspective, increase the 

 working distance of the objective from the object, and lessen aberra- 

 tion so that delicate structures invisible with ordinary means can 

 be perceived. 



Those who have seen most of Dr. Pigott's experiments entertain 

 the highest opinion of his investigations and suggestions ; but the 

 Searcher has not been successful in the hands of all who have tried 

 it, and certainly requires considerable practice and skill for its best 

 use. 



Mr. Wenham, while controverting many of Dr. Pigott's opinions, 

 has given much important information, not hitherto made public, on 

 • the construction of lenses ; and having regard to his remarkable 

 skill, and to the success of his past efforts, we may look forward 

 with hope to his rendering still further service in bringing object- 

 glasses to the highest pitch of practical perfection. 



From Mr. Carruthers we have received an account of the struc- 



" * This portion was prepared by the Secretaries. 



VOL. V. H 



