124 HOW TO SEE WITH THE MICROSCOPE. 



objective (and American observers ought to be, to a 

 man) will find the subject ably discussed in the columns 

 of the London Microscopical Journal, reference being 

 made to the celebrated controversy on the subject of 

 angular aperture between Mr. R. B. Tolles, of Boston, 

 Mass., and Mr. Wenham of London. In this discus- 

 sion, the American side of the question was ably assisted 

 by Col. J. J. Woodward, of the U. S. Army, and Prof. 

 Keith, of Georgetown, D. C. The entire controversy 

 is well worth reprinting in a consolidated form, and 

 should find an appropriate place in the library of every 

 American observer. 



No attempt will, tor the reasons given, be made to 

 discuss the subject of balsam apertures in these pages. 

 We shall, however, try and give the novice an idea or 

 two connected with balsam angles without which some 

 things which will hereafter be presented, would be 

 wholly unintelligible. 



Suppose we put a ray of light down the tube of the 

 microscope, thus reversing the usual order of things, 

 and that said pencil have an angle of 41. This pencil 

 traversing a suitable objective in position over a bal- 

 samecl mount, will find emergence into air at 90 ; equal 

 to what is recognized as 180 of aperture. Such an 

 objective would be said to possess a balsam angle of 82, 

 in other words (rejecting fractions), the balsam angle 

 of 82 is said to equal an air angle of 180. 



Now it is claimed by certain American opticians, that 

 it is possible to construct immersion lenses that are 

 capable, when worked over balsam mounts, of recogniz- 



