The Advancing Powers of Microscojnc Definition. 67 



Jan. 8, 1872. With Powell and Lealand's new inmiersion ith 

 I succeeded in showing them with the same direct condenser, in a 

 crowded form, with lines running between at right angles, forming 

 a plaid-hke pattern (square, and no longer oblique), exhibiting the 

 double set and no intervals. 



In my paper of December, 1869 (received May 21, 1869), I 

 mentioned the double row of beading, but with the yV^h Powell and 

 Lealand of 1862 I could not succeed in developing such decided 

 appearances as now described. But the beaded form was so familiar 

 with even their 1862 |th that there appeared nothing extraordinary 

 in the matter till our late President announced in July, 1869, the 

 renowned hemispheres of the Formosum, as shown by " Eeade's 

 Prism " (two months after my paper was submitted to the Council). 

 I have reason to believe that our late excellent and distinguished 

 President was in ignorance of the fact that the writer had the 

 priority. But I even then insisted upon the doiible set now beau- 

 tifully displayed by the new glasses. 



It may be remarked that a plane mirror reflects five or six false 

 images, and even a prism of reflexion which depends upon the 

 truth of its plane surface is a poor substitute for direct unreflected 

 light, which I always employ in the most delicate investigations. 



In face of these observations with the best glasses, I presume, in 

 existence (Powell and Lealand's own 1-1 6th, which I think they are 

 nnwilhng to dispose of, and their new l-50th immersion made 

 expressly for the writer), I may make bold to predict that a double 

 set of beading wiU also be discovered upon the Angulatum, Ehom- 

 boides, and other difficult diatoms. 



Besides diatoms, however, there are beaded scales of such extreme 

 delicacy as to try severely the observer's patience, as well as the 

 quality of the very best glasses now extant. The finest that I 

 have hitherto been able to resolve are the exceedingly minute 

 markings upon the ribs of the gnat's wing scale, mounted in Canada 

 balsam. Powell and Lealand's \ih. new immersion can hardly be 

 said to accomplish this feat, although it readily resolves all the 

 ribbed scales sent over by Chevalier de Cerbecq. Probably, before 

 microscopists wiU succeed in this resolution, considerable practice 

 will be required upon coarser, yet similarly mounted scales. This 

 remark leads me to make a few observations upon 



The Kesolution of Kibbed Scales. 



The advancing penetration of a microscope of the very highest 

 class now attainable, when all necessary attention is paid to the 

 correction of the aberration of the eye, as well as the eye-pieces and 

 objectives of the observer, is in no way better exemplified than in 



