BALSAM APERTURES. 125 



\u<r interior pencils greater than 41. Mr. Tolles claims 

 for some of his recent immersion objectives, balsam 

 angles as high as 120.* We have devoted a great deal 

 of time to the study of this class of objectives. 



These glasses were generally known here at home as 

 '* duplex," or four system immersions, as distinguished 

 from the older form having a single front. Many of 

 these objectives, ranging in their claims as to balsam 

 angle from 82 to 100, have passed through our hands, 

 and have been submitted to close and careful study ; 

 one object on our part being to determine, if possible, 

 whether a glass said to be of a lew balsam angle was 

 in any respect characteristically different from another 

 claiming a higher balsam angle, and in this way to 

 arrive at some determination as to the existence or val- 

 idity of the claims resting on the recognition of the 

 angle itself. 



As a result obtained from close, tedious and pro- 

 tracted observations, dating from the present date back 

 to that of the first " duplex " made, we unhesitatingly 

 affirm that it is quite possible to distinguish the per- 

 formance of a duplex objective of 82 balsam angle, 

 from a similar glass of 100, otherwise we would have 

 had no occasion to have introduced the subject at all. 



Now, in the effect of balsam aperture, we recognize 

 in the high balsam angles precisely what has been at- 

 tributed to high air apertures, namely, a decrease of 

 working distance, as the balsam angle is increased, and 



* Messrs. Tolles & Spencer are now (1880) making objectives of 120 balsam 

 aperture. 



