342 F. E. Wright — Measurement of the Optic Axial Angle 



optical phenomena to the best advantage, and that by tilting it 

 a certain angle, the interference figures can be improved mate- 

 rially. This is particularly the case with fine-grained artificial 

 preparations where, although individual crystals and cleavage 

 can frequently be obtained, they do not rest in the section in 

 the most advantageous position. Such crystals and crystal 

 plates can be tilted either by means of an axial angle appara- 

 tus for the microscope, as that described by Bertrand* many 

 years ago, or by use of the glass hemisphere of Schroeder van 

 der Kolk,+ or by the new upper condenser lens of ten Siethoff. ;£ 

 The last two methods are qualitative methods only, while that 

 of Bertrand, although quantitative, permits of revolutions only 

 in one plane. To supply the want of a universal condenser 

 lens on which angular movements can be accomplished and 

 measured in any direction, the writer has modified the Fedorow- 

 Fuess universal stage by having a brass disk, L, constructed in the 

 workshop of the Geophysical Laboratory to fit in the Fedorow- 

 Fuess stage (large model) in place of the inner ring bearing the 

 glass with cross hair (see fig. 7, page 332). Into this ring the 

 upper lens of the ten Siethoff condenser lens system is inserted. 

 The partially bevelled upper surface of this condenser lens has 

 a radius of l-5 mm , and permits, even with a No. 9 Fuess objec- 

 tive, angular movements of about 30° on either side of the nor- 

 mal. By means of a proper cylinder, N, of brass (fig. 7, page 

 332) resting on the cylinder containing the lower nicol, the 

 remaining lenses of the condenser system are raised to the 

 required distance from the upper lens. This type has proved 

 extremely useful for work with artificial preparations, since by 

 its use sections may be so placed that the most favorable meas- 

 urements possible can be accomplished with the double microm- 

 eter ocular, and in certain cases even, where the optic axial 

 angle is small, the same can be measured directly by means of 

 the universal stage angles in convergent polarized light. 



Although the measurements accomplished by the universal 

 stage methods of Fedorow are made in parallel polarized light 

 and with low power objectives, the same objectives can be used 

 for weakly convergent polarized light with Bertrand lens and 

 the position of the optic axes thus determined if it be possible 

 to bring them within the field of vision and provided they are 

 sufficiently distinct for accurate location under these conditions. 



For general work, however, with thin sections in convergent 

 polarized light, the methods requiring the double screw microm- 

 eter ocular are the most accurate and easy of application. 



*E. Bertrand, Bull. Soc. Min. Fr., hi, 97-100, 1880. 



f Schroeder van der Kolk, Zeitschr. f. wiss. Mikroskopie, viii, 459-461, 1891, 

 and xii, 188-189, 1895. 



|E. G. H. ten Siethoff, Centralblatt f. Min , 657, 1903. 



