Wright — Interference Figures under the Microscope. 19 



Art. Y. — A Modification of the Lasaulx Method for Observ- 

 ing Interference Figures under the Microscope / by Fred 

 Eugene Wright. 



Of the many plans which have been proposed for adapting 

 the microscope to observations in convergent polarized light 

 the method of Lasaulx is probably the best and is in general 

 use by petrologists at the present time. His mode of pro- 

 cedure consists in observing the image of the interference 

 figure directly, as it is brought to focus by the high power 

 objective alone, without the aid of ocular and compensating 

 lens. The interference figures thus produced, when compared 

 with those obtained by other methods, are smaller but more 

 distinct and brighter and therefore better suited to general 

 work. The weak point of the method is the time lost in 

 removing and replacing the ocular for each observation and 

 the annoyance caused thereby. 



1 The following device has been 



constructed to obviate this difficulty 

 by reflecting the light rays to one 

 side of the ocular, as shown in the 

 diagram (fig. 1), and has been found 

 in practice to answer the purpose 

 well. In the writer's microscope the 

 small reflecting apparatus fits in a 

 narrow slot in the microscope tube 

 between the base of the ocular in 

 position and the iris diaphragm and, 

 like the upper nicol prism, can be 

 inserted or drawn out at will. The 

 mechanical construction of the device 

 is so simple that a good mechanic 

 kMjjB.j. can make it without trouble. It has 



; yL ! ■■gssssg i t D been observed that the loss of light 



on reflection is not sufficient to 

 decrease the brilliancy of the inter- 

 ference phenomena appreciably. 



With this appliance attached to the 

 microscope the passage from plane 

 Vertical section of upper parallel to convergent polarized light 

 part of microscope tube show- f PJ ^-ji v P flW pr q bv irmprtino- rhP%P 



ing position of reflecting when ^ easil J enectea oy inserting me re- 

 inserted. 0, ocular; R. A., fleeting apparatus and observing then 

 reflecting apparatus; B. L., the interference phenomena at one 

 position for Bertrand lens; • i f f i OP11 l ar Th P ocular re- 

 I. D., iris diaphragm. S1Q ? 0I Ine OCUiai . J. lie OCUldl ie 



mams thereby in place and practically 

 no time is wasted during the operation. 



Sliding Stop Diaphragms as Substitutes for the Iris Diaphragms 

 in JPetrographic Microscopes. 



In microscopic work it has been found by petrologists that 

 many optical phenomena are rendered more apparent by the 



