1895,] Mineralogy. ; 269 
Klein’s Lens with Micrometer.—Becke” has designed an 
attachment to fit over the Czapski ocular like the common form of 
analyzer, to determine the size of the optical angle in very small crys- 
tals when the section is approximately normal to a bisectrix (Mallard’s 
method). This device has fitted into its upper part, so as to be adjust- 
able by friction, an aplanatic lens magnifying eight times. Below this 
is an ocular micrometer which can be raised or lowered by means of 
two heads on the outside of the attachment. The interference figure 
observed with this instrument is not the one obtained by the objective 
alone but the one formed in the upper eye point of the microscope 
above the Ramsden’s ocular. Before using the attachment the minute 
crystal is centered and the diaphragm of the Czapski ocular closed 
until the crystal alone is visible. The Klein lens is now adjusted over 
the ocular till the objective diaphragm is visible, when the interference 
figure may be distinctly seen. The micrometeris now adjusted to read 
without parallax. The constant K of the combination (in Mallard’s 
formula sind=4. d) for a given length of tube is obtained by measur- 
ing d in the case of several sections normal to a bisectric whose optical 
angle has been determined by an axial angle apparatus. The attach- 
ment can also be used to measure the azimuth of any point in the inter- 
ference figure with reference to cleavage or twinning line, etc. The 
long middle line of the micrometer is placed in the azimuth of the 
point to bedetermined. On introducing the Bertrand lens and slightly 
altering the length of the tube the image of the section appears. The 
stage is now revolved until any direction desired is brought parallel to 
the micrometer line and the angle measured. This device is useful to 
determine the changes in optical orientation in different parts of a 
erystal individual and to determine the position of the optic axes in 
the twinned lamellæ of the plagioclases. 
Wm. H. Hosss, 
Min. u. petrog. Mittheil., xiv, pp- 375-378, 1894. 
