80 
The American Geologist. 
February, 1896 
This apparatus is briefly and modestly referred to by Prof. 
Lacroix in the preface to his work La Mineralogie de 
la France et de ses colonies , (1893, Paris), but it has never 
had adequate description. It is based on the suggestion of 
Mallard for the construction and use of an “ oculaire a flls 
mobile,”* and it is represented by figure 1. It can be adjus¬ 
ted to any microscope, since in use it is inserted in the top of 
the body-tube in the same manner as the common ocular. It 
gives the optic angle measured in air. It contains a Bertrand 
lens at D for magnifying the interference figure, but if the 
microscope have one already fitted to the lower end of the 
upper tube, it is often better to remove the lower part of this 
instrument, and with it the Bertrand lens since the lower 
Bertrand lens increases the interference figure, by which the 
delicacy of the measurement is also increased. When thus 
separated the lower lens T of the detached eye-piece should 
be screwed into the lower end of the remaining eye-piece of 
the goniometer. Thus adjusted it is to be used in conjunction 
with the Bertrand lens belonging to the microscope .\ 
When the interference figure is perfect, showing the loci of 
the optic axes, and the axial goniometer is in position, the 
revolution of the graduated wheel B, which is independent of 
that of A, will serve to bring one of the movable threads into 
the central point of one of the optic axes. Then by the pro¬ 
per rotation of A, which also carries the graduated rim B, this 
thread can be made to move slowly across the field and to 
occupy a similar position over the other optic axis. The 
number of revolutions and parts of revolutions can be recor¬ 
ded by comparison with the zero point of the vernier-scale 
at E. 
Example for the measurement of the optic angle. 
In figure 2 let O be the point of crossing of the optic axes 
in a thin section of a biaxial mineral AH. Let 2Y represent 
the acute axial angle, and 2E the same angle measured in 
air. The angle COD is one half of 2E, or E. The distance 
between C and I) is the sine of the angle E. It is evident 
that the value wanted is the distance 2sinE. 
*Bul. Soc. Min. de France, V, (1882), p. 78. 
fAs made by Ivan Werlein, 76 rue Cardinal Lamoyne, Paris, without 
the Bertrand lens and the lower ocular F, fig. 1, the cost is 120 francs. 
