20 Wright — Interference Figures under the Microscope. 



use of diaphragms both below and above the condenser and 

 objective lens systems of the microscope ; that, by closing 

 gradually the lower iris diaphragm not only is the definition 

 improved but also methods based on the relative refractive 

 indices of adjacent mineral sections, as the Becke line method, 

 are materially bettered ; and that, by closing the upper dia- 

 phragm in like manner, interference figures in convergent 

 polarized light can be observed from minute mineral plates 

 which would be otherwise totally obscured by surrounding 

 minerals. Ordinary petrographic microscopes, however, are 

 not fitted with such iris diaphragms and the observations suffer 

 accordingly. 



The following sliding-stop diaphragms have been used as 



substitutes for the iris diaphragms and are of such nature that 



anyone with mechanical ability can construct similar ones for 



2 any given instrument. The 



upper stop diaphragm (fig. 2) 



is made of brass similar in 



general outline to that of the 



Bertrand lens, — the observa- 



Oo 



C tions in convergent polarized 

 Sliding-stop diaphragm— a substi- light being made after the 

 tute for the upper iris diaphragm of Lasaulx method, either by re- 



a petrographic microscope. ^^ ^ ^^ or hj ^ 



of the reflecting apparatus described above. The stop dia- 

 phragm consists of two parts ; A, the body, which is so con- 

 structed that, when inserted, the drilled hole occupies the 

 center of the field and provides thereby one stop ; and B, a 

 thin strip of brass which fits in wedge-shaped grooves milled in 

 A and into which the remaining stops are drilled. A short 

 screw C provides a simple handle by which to grasp the appar- 

 atus. In actual practice the mineral section to be tested should 

 first be sharply focussed and centered, the stop diaphragm 

 then inserted and by means of an appropriate aperture the light 

 from all other minerals in the section shut off, after which 

 the observations in convergent polarized light can be made. 



The lower stop diaphragm is of still simpler construction 

 and consists of a strip of brass with variable sized stops not 

 unlike those of the above apparatus. The strip is fitted into, 

 and guided by two wedge-shaped pieces of brass screwed to 

 the lower end of the cylinder containing the lower nicol. 

 The lower stop diaphragm has been found particularly useful 

 in the application of the refractive liquid method to mineral 

 fragments.* 



Geophysical Laboratory, 



Carnegie Institute, Washington, D. C. 



* Compare Tschermak's Miner, petr. Mitteil., 1901, xx, pp. 289 and 251. 

 Strips of thin cardboard with appropriate perforations can be used as tem- 

 porary substitutes for both iris and sliding stop diaphragms. 



