416 F. E. Wright — New Ocular for Use with the 



attachments into one ocular is novel and of sufficient general 

 interest to warrant brief description. 



The ocular is represented in tig. 1, and consists essentially 

 of a metal holder, which is inserted in the microscope tube in 

 place of the ordinary ocular and into which in turn a positive 

 Kamsden ocular* is introduced at A and certain plates 

 mounted in metal carriages, a, b, c, are inserted at B. Cross 

 hairs are attached to the base of the tube A and are practically 

 in the same plane with the upper surfaces of the sliding plates 

 a, b 9 c, with the result that on focusing the "Ramsden ocular on 

 the cross hairs, the divisions marked on the plates a, h, c, are 

 also in focus and their relative movements can be read off 

 directly. "With the above arrangement the optical constants 

 required can be measured directly by means of the three plates. 



Fig. 1. 



Fig. 1. New ocular with accompanying plates a, 6, c, to be used in 

 measuring the birefringence, the optic axial angle, and extinction angles of 

 minerals in the thin section or in powder form. 



Plate a (fig. J, fig. 2a, b) is a combination quartz wedgef 

 35.3mm ] on g anc [ iQmm w id e> anc j consists (fig. 2a) of a 

 quartz wedge cut parallel with the principal axis (direc- 

 tion of elongation — c ) '5 imn thick at the thin end and 

 •89 mm at the thick end, its pitch being, therefore, about 

 6° 16'; and (2) of a quartz plate with direction of elonga- 

 tion a of same length and width and -56 mm thick. If these 



* An ordinary Huyghens ocular can also be used, provided the plates be 

 inserted in the focal plane of the aplanatic eye lens. 



f Compare F. E. Wright, Tscherm. Min. Petr. Mitteil., xx, 275, 1901; Jour. 

 Geol., x, 33-35, 1902. 



