106 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Microscope, bears on its under surface two vertical tubes containing two 
Isicol prisms N c and Np, one of which N c is permanently arranged 
for observation by convergent light, i. e. with the usual two convergent 
lenses, whilst the other Np bears the usual lens employed in observa- 
tions by so-called parallel light. Both of these polarizers, together with 
their lenses, can be raised or lowered independently of one another by 
means of suitable forked two-pronged levers H c and H p. 
\\ hen changing from one form of illumination to the other, the 
Nicol prism last in use is pushed down, the plate S is slid in its groove 
so as to bring the other Nicol to the centre of the stage, and this second 
Nicol is then raised by its lever into position beneath the mineral 
section. These four stages in the process are shown in the diagrams 
I. to IV. (fig. 5), where I. shows Nicol N c in its highest position, 
Fig. 5. 
beneath the section ; Nicol N p in its lowest position on one side, not in 
use. II. Nicol N c in its lowest position beneath the section ; Nicol N p 
in its lowest position on one side, as before. III. Nicol N c in its lowest 
position, pushed aside ; Nicol N p in its lowest position beneath the 
section. IV. Nicol N c in its lowest position, pushed aside as before ; 
Nicol Np in its highest position beneath the section. 
The catch or stop at A can be turned aside to permit of both 
polarizers being pushed aside for observation by ordinary light. Dr. 
Wiilfing has the apparatus attached to a Fuess No. 1 large Microscope. 
It would require special fitting to other patterns. The apparatus is 
made by Zimmermann, Mechaniker, Hauptstrasse, Heidelberg, and costs, 
without Nicol prisms and lenses, 60 marks. It will be observed that 
two polarizers are required. 
Bulloch’s improved Filar Micrometer. — Fig. 6 shows this in- 
strument, of which a short description was given in this Journal for 
1886, p. 132, in which we described it as having a second screw, worked 
by a milled head at the opposite end to the micrometer-screw, which 
moves both sets of lines together, so that it is possible to set the 
graduated screw-head at zero for any particular measurement. 
