412 F. E. Wright — New Petrographic Microscope. 



angles to each other. This cap fits the microscope tube and is 

 inserted in place of the ocular. By means of the lens "a" 

 the field is focused in the plane of the slides and any por- 

 tion singled out for examination. 

 Fig. 4. Because of diffraction phenomena 



the aperture should not be made 

 ; — 19HHHK ~ less than *5 min in diameter, but even 



with this restriction, and with the 

 -<X ordinary objectives, 3 or 4 mm focal 



length, grains not over -01 mm furnish 

 ^JI ^jc. good interference figures which or- 



dinarily would be completely over- 

 shadowed and not discernible if 

 adjacent light were not excluded. 



Experience has shown that the 



effects are still clearly recognizable 



Fig. 4. Device for cutting ^ the diaphragm is at a distance of 



down the field when interference not over 5 cm from the eye, and for 



figures from small grains are convenience sake, therefore, this 



being observed as they form m i • -i • y , j • , Z ^ 



the objective itself (Lasaulx diaphragm was inserted just below 

 method). Two sets, Si and s 2 , the ocular. The usual round disks 

 of two plates at right angles with small aperture supplied with 



and sliding in grooves permit mi v ro( , ponp( , q PrV P +1ip ^flmp nnr- 

 the observer to cut off the light miC1 °SCOpes sei v © .lie same pm- 



from any part of the field. Be- pose but are less convenient. 



fore observing the interference Before stopping down the field 

 figure, the image from the objec- b th diaphragm Y just below 

 tive is first brought to comci- ,/ . K , & # .j 



dence with the plane of the iris the Bertrand lens, the image plane 



diaphragm by means of the field from the objective should on the 



lens -'a." same principle be brought to coin- 



cide with the plane of this dia- 

 phragm and the desired mineral section isolated by shutting off 

 light from the adjacent grains. To accomplish this readily, a 

 small lens, L, -Rg. la (19 mm focal length) has been introduced in 

 the present microscope above the Bertrand lens, and in conjunc- 

 tion with the ocular serves the purpose of bringing to sharp 

 focus the image picture in the plane of the Bertrand lens iris dia- 

 phragm, in accord with the principle noted above. In place of 

 this small auxiliary lens, the writer has heretofore used a lens of 

 long focal length and viewed the Bertrand lens diaphragm 

 directly from the top of the tube. The new arrangement is 

 more convenient, however, and obviates the necessity of remov- 

 ing the ocular before viewing the interference figure. The 

 lens L swings on an axis and can be instantly thrown out of 

 the field. A small spring with pointer automatically indicates 

 the correct position of the lens when thrown into the field. 

 The Bertrand lens diaphragm ordinarily supplied with micro- 

 scopes is of little value in the observation of interference 



