ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 505 



which, in connection with the sliding ocular tube, permits of different 

 magnifications of the interference figure, an arrangement already adopted 

 on several well-known Microscopes. In the present Microscope the focal 

 length of the Bertrand lens (55 mm.) has been so calculated that the 

 initial magnification of the interference figures can be varied from 

 0'81 to 1*90 diameters. The ocular itself magnifies this image in turn 

 eight-fold, so that the resulting magnifications range from above 6 '5 to 

 15 '2 diameters. The fact that the upper nicol intervenes between the 

 objective and Bertrand lens limits very considerably the range of magni- 

 fication possible by the Bertrand lens. An iris-diaphragm is introduced 

 directly below the Bertrand lens, and slides up and down simultaneously 

 with it. This diaphragm is opened and closed by means of the pin V 

 (fig. 5-4), which is connected with the diaphragm itself by means of 

 pin-and-ratchet movement. 



7. A second iris-diaphragm is introduced at CI (fig. 53), directly 

 below the ocular, and is used in connection with the observation of inter- 



jt* i i 



Fig. 56. 



ference figures by the Lasaulx method without the Bertrand lens. To 

 be of service in this connexion, the iris-diaphragm should be located 

 precisely in the image-plane from the objective, as was emphasized 

 especially by Czapski in 1891, for in that plane alone can light be ex- 

 cluded from adjacent minerals in the thin section. To realize satisfac- 

 torily this condition, the author has heretofore used the cap and stop 

 indicated by fig. 56, with the two sets of slides, 8j and S 2 , at right 

 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 portion singled out for examination. Because 

 of diffraction phenomena the aperture should not be made less than 

 0*5 mm. in diameter, but even with this restriction, and with the ordi- 

 nary objectives, 3 or 4 mm. focal length, grains not over 0*01 mm. 

 furnish good interference figures which ordinarily would be completely 

 overshadowed and not discernible if adjacent light were not excluded. 



Experience has shown that the effects are still clearly recognizable if 

 the diaphragm is at a distance of not over 5 cm. from the eye, and for 

 convenience sake, therefore, this diaphragm was inserted just below the 

 ocular. The usual round disks with small aperture supplied with 

 Microscopes serve the same purpose, but are less convenient. 



