Objectives and Eye-pieces.
In the manufacture of our objectives only such glass is employed as has been subjected to the most rigid scientific tests. By these the exact index of refraction and the exact degree of dispersion of the glass are determined, and with these data available it is possible to very perfectly correct both spherical and chromatic aberrations while still making use of wide angular apertures in the objectives.
The precise mathematical calculation, combined with accurate systematical working and testing methods, make it possible for us to guarantee our objectives to be all of equal and excellent quality.
Every objective before leaving our hands, is subjected to the most careful test, and only such lenses as are of the highest grade, are sent out.
For the past seventeen years we have used glasses manufactured by Schott & Co. of Jena. This glass has many points to recommend it for the construction of optical instruments, and only such kinds are employed by us, which have for many years been thoroughly tested as to their durability.
Our lenses are therefore absolutely permanent. Objectives of the earlier type which have become cloudy, we shall gladly repair.
The three illustrations given above afford an idea of the plan of construction of our achromatic objectives.
The first figure represents the plan of our low power objectives, and it will be noted that they consist of two doublets, or triplets each carefully corrected.