Tlie Advancing Powers of 3Iicroseopic Definition. 65 



appearance (structureless as regards beading), the 1-5 0th immer- 

 sion of Messrs. Powell and Lealand displays a rich profusion of 

 beading. 



I have also succeeded in displaying the same results with the 

 Lealand ith and Wray ith by means of sufficient amplification 

 and accurate spherical and achromatic corrections. 



The faltering steps by which microscopists have during the last 

 twenty years groped their way to their present magnificent results 

 must ever be instructive, as developing the grades by which a dim, 

 difficult, almost desperate definition has given place to a brilliant, 

 certain and satisfactory evolution of pre\aously unknown forms. 



The question as it now stands ajjpears to warrant the conclusion 

 that all memhranous structures of the scales of ivinged and also 

 imwinged insects are approximately striated, the strise in the two 

 layers being either parallel, inclined to each other, ivavij or radi- 

 ating either in one set or both. I use the term approximately as 

 denoting the first apj)roximation attainable to ordinary definition. 

 As the definition advances, these striae show in different planes, 

 become ribs ; and most probably from either difiraction or polariza- 

 tion or both, being in difierent planes, exhibit complementary 

 colours red and greenish blue, according to the compound character 

 of the illuminating ray, or if monochromatic light be employed, 

 then shaded more or less. The next step in a more advanced defi- 

 nition is the resolution of these ribs and intercostal spaces into 

 beads, molecules, or agglomerated particles. When these lie in 

 difi'ereut planes, and the corrections are brought to the utmost per- 

 fection at present attainable, they assume respectively difierent 

 colom's, usually ruby or pink, red and sea green, or sapphire blue. 

 The assemblage of these minute little ruby and sapphire like bodies, 

 densely crowded within the ribs (if not actually forming them), is 

 one of the prettiest autographic pictures (of what the microscope 

 may yet be expected easily to perform with proper corrections) with 

 which I am acquainted. Indeed, I may perhaps be excused for 

 venturing the opinion that had the marvellous glasses of to-day 

 been employed a few years ago, many of the microscopical papers 

 (^innocently contributed) would never have been penned; at this 

 moment they seem but a monument of transitional science.* 



This paper would be extremely incomplete were I to omit to 



* For the information of the Fellows who may be disposed to order Messrs. 

 Powell and Lealand's l-50th immersion objective (price 30 guineas), I may state 

 that it is incumbent to use a covering glass of about 3-lOOOths of an inch thick : 

 but that I have found the very clear talc formerly employed by Pritchard (several 

 of whose slides I possess, as also his wonderful doublets of gJ^th of an inch focal 

 length) answers remarkably well with the new immersion objective. 



It is interesting to observe that these doublets, though only possessing an 

 aperture of a very few degrees, develop the exclamation marks in some extremely 

 old scales of Lepidocyrtus curvicollis. These appearances arc therefore not dependent 

 upon hrife aperture, which appears to me a very significant and striking fact. 



