1887. ] MICROSCOPICAL JOURNAL. ‘109 
38. DisTEMMA. 
a. Body long, gibbous behind, changeable; marine . . . . . raptor. 
6. Body cylindric, long; toes one-third the entire length . . . Collinsiz. 
c. Body gibbous, long, slender; foot long, toes minute . . . dadbcatum. 
MIROSCOPICAL TECHNIQUE. 
Lighton’s Analyzing Diaphragm for Polariscope. 
By WILLIAM LIGHTON. 
LEAVENWORTH, KANSAS. 
The following described piece of apparatus has been found by me to be of 
great help in the study of crystallography, and I have thought that a des- 
cription of it to working microscopists through your ,Jowrzal would help 
along the good cause of microscopy. 
We will suppose that the polariscope as ordinarily used has been placed in 
position, the poJarizing prism below the stage and the analyzing prism above 
the objective. 
The apparatus consists simply of a cap with movable diaphragm placed 
over the eye-piece, as illustrated in figures 1 and 2. Fig. 1 is a sectional view 
and Fig. 2 a top view of cap of eye-piece. 
The letters in both draw- 
ings refer, as will be seen, 
to the same parts. 
Let A indicate the axis of 
tubes B, Big. 1, the eye- 
piece ; C, the cap of eye- 
piece. The apparatus con- 
sists merely in a diaphragm 
plate, D, swinging from 
right to left on the pivot I, 
ies a.) This’ motion’ is 
given by placing the finger 
at the knob L. The amount 
of motion is controlled by 
the two small studs G. The 
diaphragm is pierced by a 
small hole H, one eighth of 
an inch in diameter. E is 
a screw in top of post I, 
Fig. 1, holding diaphragm 
in place. Fis apex of cone 
of light formed by the image of source of light passing through the eye-piece. 
Now if the diaphragm be so adjusted by sliding the cap upon the eye- 
piece that it will be on a level with this point of light a very interesting se- 
ries of optical effects will be observed. 
The small studs, G, should be so placed that when the diaphragm is swung 
to the right or left the sides of hole, H, will just cut the axis of eye-lens 
(apex of cone of light). 
I will mention a few of the sights seen by its use as described above. In 
no case were the prisms of polariscope revolved. 
A crystal of chlorate of potash was selected which, upon simply revoly- 
