118 C. A. Steinheil Sohne, Mtinchen. 



Plane test glasses, 



for testing perfectly plane surfaces. Glass No. I must fit II and III, 

 and II must fit III with perfect exactness. 



Piano-parallel Glasses 



absolutely exact, in various sizes and thicknesses. According to thick- 

 ness classified in 



ordinarily thick, viz. 3 to 6 millimeter, 



thin 1 to 3 



thinest glasses . . about- 1/3 



Piano -parallel Mirrors, 



intended, in particular, for reflecting galvanometers, very thin, about 

 1/3 millimeter, in case. The piano -parallel mirrors are silvered on 

 one side and form mirrors acting through the glass. The silver is coat- 

 ed with a protecting layer. 



Rectangular crown glass prisms 



with square cathetus surfaces, with sharp edges, absolutely correct 

 angles and without pyramidal defects. 



Triple Rutherford Prism. 



Example of dimensions: Aperture of the polished surfaces 27 to 

 34 millimeters (12'" to 15'"), consisting of a thallium prism of 90 

 cemented between two crown glass prisms of 20, which not only pro- 

 tect the surfaces of the thallium from oxydation but also renders it 

 possible to make the refracting angle of the thallium prism so large 

 that without the addition of the crown glass prisms total reflection 

 would take place. 



Aplanatic Magnifying Lenses. 



These are so formulated as to possess considerable focal 

 distance and to give an undistorted image achromatized in the central 

 and extraaxial parts of the field (vid. M. Schultze's paper in Schultze's 

 Arch. f. mikr. Anat. V. II, 1866.) 



Aplanatic lens of 95 millimeter (42'") equivalent focus: linear 

 magnification about 2*/ 4 ; large flat field suitable for examining maps etc. ; 

 in polished mounting. 



Set of 6 aplanatic magnifying lenses with holder in case. 

 The mounting consists of a simple blackened ring. 

 Equivalent foci: 61 41 27 18 13 9 millimeter, 



27 18 12 8 6 4 lines, 

 Magnifications 3.5 5.3 8 12 16 24 

 Set of 5 aplanatic lenses in folding mount; in case: 

 Equivalent foci: 41 27 18 13 9 millimeter, 



18 12 86 4 lines, 

 Magnifications 5.3 8 12 16 24 



