26 CORRESPONDENCE. 



virginlca or Campanula sylvesiris; the corpuscles which are seen 

 floating in the fluid have the appearance of little spheres, so clearly 

 defined that it seems impossible that human skill can ever contrive to 

 make an optical instrument which will show more or better than our 

 microscopes when well and judiciously employed; and yet it is 

 evident that there is something more to be seen, because those spheres 

 are not only dragged by the force of the stream in which they are im- 

 mersed, but they have their own and individual motion ; so they must 

 have some sort of propellers as yet unknown and unseen. 



I am, sir, yours truly. 



Chevalier Hcyttens de Cerbecq. 



The Angular Aperture of Tolles' ith. 



To the Editor of the ' Monthly Microscojrical Journal.'' 



Boston, Mass., November 15, 1871. 



Dear Sir, — Permit me to reply to the question of your corre- 

 spondent B. in the November issue of your Journal. The angular 

 aperture of the Tolles' objective (" nearly |th") used by Dr. Wood- 

 ward as " immersion" varies from 140°, at " uncovered," to 170°. How 

 Dr. W. used it I do not know. Used " dry " (ith) it ranges from 110° 

 to 170°. With another front "system" it is a ith 110° up, and up- 

 wards. The aperture as well as the power increasing as the lenses are 

 brought nearer together. 



A few words may be added on B.'s remarks on aperture. He refers 

 to the " confusion which exists in the minds of most microscopists 

 about the classification of glasses," and conveys the idea, though he 

 does not express it in words, that angular aperture is not taken into 

 consideration. That such is the case with some makers, and some 

 owners of microscopes, is undoubtedly true ; but so far as my experience 

 goes it is not the case with " microscopists." With them aperture is as 

 much considered as focal length ; and I find that with " microscojiists," 

 " focus and aperture are in fact both essential factors in the denomi- 

 nation of an object-glass." This subject was elaborated by Dr. W. B. 

 Carpenter in the first edition of ' The Microscope and its Kevelations,' 

 published more than sixteen years ago — a work that is in the hands of 

 most microscopists, and which B. has probably seen. Since that time, 

 however, skilful opticians have made object-glasses of veiy high 

 angular aperture and retaining greater penetrating power than such as 

 were known to Dr. Carpenter tlieii, and consequently better adapted for 

 purposes requiring that quality than he jirobably expected they ever 

 could be, and the same glasses retain the advantages derived from the 

 great aperture. All opticians have not progressed in this direction, 

 but some continue to make their lenses as they did twenty years ago. 



As a trip to the United States is now a favourite one with English- 

 men, I hope that B. will make us a visit. Americans have been re- 

 l)orted as having "made some very good instruments," and it will afford 



