38 Experiments on Angular Aperture. 



The convenience of using an incidence of only about 50° to 60° 

 on eacli side of the axis, instead of nearly 90°, is evident enough. 

 That this plano-convex lens fixed in the centre of the stage, 

 perhaps preferably made achromatic, wiU be utilized as a condenser, 

 there seems no doubt. In my own hands it seems to doubly de- 

 monstrate difficult tests. 



Certainly the use of immersion condensers is abundantly in- 

 dicated in the above simple experiments. 



Boston, Mass., U.S.A., May 2ith, 1871. 



Db. Henry Lawson, 



Editor. Boston, 3fay 25th, 1871. 



Dear Sir, — I yesterday mailed to your address a paper by Mr. 

 Tolles on immersion objectives. I now wish to make one correction 

 in that paper. The angle of the rays entering the objective with this 

 arrangement is 110° instead of 100° as written. This makes the 

 angle nearer to Dr. Pigott's statement, and farther from Mr. Wen- 

 ham's. Please make the change when printing the paper, which I 

 hope is in season for the July issue. 



Respectfully, 



Chas. Stoddek. 



