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III. — The Advancing Powers of Microscopic Bejiniiion. 



By Dr. Eoyston-Pigott, M.A. Cantab., F.C.P.S., F.E.A.S., M.E.I., 

 F.E.M.S., formerly Fellow of St. Peter's Coll., Cambridge. 



PaH 11. 



(^Continued from page 68.) 



It seems unnecessary to describe further the comphcated beaded 

 system of insect scales distinguishable by the most perfect glasses 

 now made, notwithstanding the very beautiful specimens sent over by 

 The Chevalier de Ceebecq, which I strongly recommend to the 

 attention of oiu* Fellows. I therefore pass on to notice more particu- 

 larly " advancing powers of definition " illustrated by Mr. Slack in 

 his interesting paper on the Pinnulariae, which gives a most valuable 

 confirmation of my views as to the inadequacy of the old-fashioned 

 glasses to deal mth the higher order of definition. As I had ex- 

 perienced considerable difiiculty in developing the beading of Pin- 

 nulari^ with an 1862 ^th of Powell and Lealand four or five years 

 ago, I was pleased to receive a statement from Mr. Slack, that with 

 his new immersion ifh of Powell and Lealand he had completely 

 succeeded in resolving the costm into component beading : at this 

 time I did not possess the new glass, x^nd I very much question 

 whether old-fashioned glasses will resolve them now. Speaking of 

 this beading, Mr. Slack sagaciously observes (even when using the 

 new ^ih) in his paper : — 



" When the best has been done with any objective, it becomes 

 evident that a slight increase of the difficulty, from greater minuteness 

 of structure, would render it invisible and make the surface look 

 plane." 



It is noteworthy that to most glasses the interspaces between 

 the beads of the Formosum and of the Podura curvicoUis Test- 

 scale do look quite ])lane and free from structure. 



" An examination of the Pinnularite on Moller's type slide, led to 

 the belief that the costfe of such diatoms as P. viridis, nohilis, &c., had 

 been misunderstood. That instead of broad, insolvable ribs, a truer 

 view exhibited fine lines of beads springing from the median band." 



Those who wish to succeed, with inferior glasses, to develop 

 Podura and Lepisma beading composing the ribs (corrugations or 

 rumplings seem terms rather indistinct to give a good idea of truly 

 formed ribbings), would probably be assisted by practising the re- 

 solution of the ribs shown by the Pinnidariae, and such diatoms as 

 Surirella gemma. Mr. Slack also says : — 



" P. NOBiLis. — Very difficult except in lucky parts and valves. . . . 

 Median band beaded in complicated pattern. 



