No. I.] SPERMATOGENESIS OF BATRACHOSEPS. J 



Optical Methods and Appliances . 



No one who has not used an oil-immersion substage con- 

 denser can have any adequate idea of the value of such an 

 appliance. In the study of the finer structures of the cell it is 

 of equal importance with the apochromatic objective, and its 

 value cannot be overestimated. It is my opinion that a correct 

 and proper view of the structure of the protoplasm cannot be 

 had without the oil-immersion condenser, and that opinions 

 based on observations made without this condenser must neces- 

 sarily need to be reconsidered. An achromatic, or better yet 

 an apochromatic, condenser should be used, and the immersion 

 oil should be slightly denser than the oil used on the objective, 

 in order to counteract the thickness of the slide. This can 

 readily be accomplished by warming the oil for a short time, 

 or by using oil that has become thicker from ordinary exposure 

 to the air. The oil-immersion condenser increases the sharp- 

 ness of the image and brings out details not otherwise visible. 



The images were studied with artificial light alone, a filter 

 of cyanin and methylene blue being interposed between the 

 Welsbach incandescent gas burner and the substage condenser. 

 For a more detailed account of this light see Zeitschr. f. tviss. 

 Mikr., Bd. XIV, pp. 444-447, 1897. The use of this light 

 enables us to work continuously, independent of sun and 

 clouds, day or night, with the same strength of light and with 

 the same facility. This light has also a great advantage in 

 not injuring the eyes, not even tiring them perceptibly after 

 ordinary work. The light has also another advantage in that 

 it differentiates structures which are not differentiated by stain- 

 ing. For instance, by the daylight and after staining with congo 

 no differentiation was had between the linin granula and the 

 cytoplasmic granula, but with the artificial light the linin gran- 

 ula were seen to be of a gray or brown color, while the cytoplasmic 

 granula were stained pale red. A better image can be had with 

 this artificial light than with the best white-cloud light, and the 

 image is also perfectly steady, which it never is with even the best 

 cloud light. The sections were studied with Zeiss Apochromats, 

 3 mm., Ap. I, 40, and 2 mm., Ap. i, 40; Oc. 12 and 18. 



