SUPERLATIVE WIDE-ANGLED OBJECTIVE. 143 



apertures there is a loss of distance. Now I propose 

 to discuss this " loss " from a practical point, and in so 

 doing shall state facts, including names and dates. 



On the 22d of March, 1878, I received from Messrs. 

 C. A. Spencer & Sons, a one-tenth objective of high 

 balsam angle. This glass was made to my order, and 

 for the Hon. J. D. Cox, of Ohio. Before sending it to 

 its destination I " fought it." Its performance was as 

 follows: Worked over the balsamed Moller probe 

 plate, on the Zentmayer histological stand, with the 

 mirror-bar swung to 31 from axis, I saw distinctly the 

 striae of Nos. 18 and 19 of said plate, nor had I any 

 doubt as to thus seeing the No. 20, the illumination 

 being a common small coal-oil lamp. The light was 

 taken direct from the mirror. 



Worked over the same Moller plate, but with the 

 Wenham reflex illuminator (an instrument devised for 

 shutting out all pencils having air transmission), I 

 saw the transverse striae of the No. 20 of said plate 

 handsomely using powers from 1,000 to 4,000 diam- 

 eters. 



Tested over a fine slide of English podura, using 

 ordinary illumination, and nearly central, I had the 

 finest display I had ever witnessed. 



Worked over navicula angulata, with "dead central " 

 illumination,, it gave me instantly the markings very 

 handsomely. 



Worked over a dry mount of amphipleura pelliicida. 

 These shells surrendered at discretion ; ordinary oblique 

 illumination. Substituting a balsam slide of this 



