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. [Illustration] 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. The central figure shows the construction of our high power dry objectives. A hemispherical front lens is combined with two doublets or sometimes triplets. The front lens is the chief magnifier of the combination, while the other lenses serve to correct the various aberrations. The Oil-immersion, represented by the last illustration consists of a front lens, hemispherical, behind which is a meniscus, which is in turn followed by a doublet and a triplet, these latter acting as the correcting lenses of the combination. We manufacture both =Achromatic= and =Apochromatic objectives=. They differ in that the glasses made use of in the apochromatics and the manner in which they are combined permit a more perfect correction of chromatic aberration. This advantage is not gained without a certain sacrifice of simplicity in construction; by avoiding the use of flint glass having a high refractive index and substituting materials to take the place of crownglass. The apochromatics as a matter of fact do resolve the fine markings of test objects (butterfly scales and diatomes) somewhat more clearly than the achromatics, but the difference is slight and in ordinary stained microscope preparations is hardly detectable. The correction of both achromatics and apochromatics is complete. The ordinary Huyghenian eye-pieces are consequently well adapted for use with the objectives of either construction. Only with the highest powers is it desirable to make use of the so-called "compensation" eye-pieces. The achromatics and Huyghenian eye-pieces are also well adapted to the requirements of photomicrography, special objectives being unnecessary for this purpose. This statement is substantiated by the excellent results obtained with our achromatic objectives, as shown in the photomicrographs accompanying our brochure on Photomicrography:--"Anleitung zur Mikrophotographie".