16 



The Optical Quality of Mr. Tolls h Objective. 



More Balsam-Angle. 



By increasing the angle of tlie back-and-middle pencil of course 

 more balsam-angle of the objective would be obtained, at the same 

 time using more clear opening of the (this) front. But the hmit 

 of angle of the inner systems being attained, not that gains for us 

 the limit of possible balsam-angle, though considerably beyond 90° 

 could be gained thus. The front can be made to play a part it 

 does not here ; helping to a still further increase. 



Place of Adjustment for Maximum Angle. 



A case of reduction of balsam-angle by mere change of position 

 to closed is shown in Fig. 2. The same front is represented in 



