HERBERT SPENCER 1/15" BALSAM IMMERSION OBJECTIVE WITH CORRECTION COLLAR AND LABELED WITH A BALSAM ANGLE OF 115^o

Circa 1875

Shown below is a rare Herbert Spencer oil immersion objective labeled with its Balsam Angle (B.A.) of 115^o corresponding to a numerical aperture of 0.91

Robert Tolles invented homogenous balsam immersion objectives in 1871, but homogenous oil immersion using Cedarwood oil was first suggested by Abbe in 1877 and this was quickly adopted by all the major makers shortly thereafter. Since the Balsam angle only applied somewhere between 1871 and 1877, or perhaps a little later, objectives with the Balsam Angle inscribed are very uncommon.