40 



MICROSCOPY OF THE LIVING EYE 







jugate foci in relation to the collecting system c. The focusing 

 lens, FL, had a large aperture and was not achromatic, so that the 

 beam at f showed chromatic aberration ; and it had another 

 defect : f and p being conjugate foci, and an image of the spiral 

 filament of the lamp being focused into the slit at p, a second 

 image of the spiral filament was again reproduced in the beam at f, 

 i.e., the beam at f was not homogeneous unless a homogeneous 

 light-source were used, such as the glowing end of one carbon of an 

 arc-lamp instead of a spiral tungsten filament. These defects 

 of the beam at f — in respect of chromatism and light-dis- 

 tribution — were subse- 

 quently corrected in this 

 way : As more efficient 

 filament-lamps came on 

 the market they could be 

 burnt at a greater inten- 

 sity, and this allowed of 

 the focusing lens having 

 a much smaller aperture 

 so that it could be made 

 in achromatic form. The 

 second error, resulting 

 from the ultimate pro- 

 jection of an image of 

 the spiral filament into the beam at f, was overcome by Vogt's 

 arrangement of the apparatus ; he had the lamp bulbs made 

 so that the spiral filament, s', could be pushed closer to the 

 collecting system, C, until the image of the filament was pro- 

 jected on to the plane of FL', i.e., s' and FL', instead of s 

 and p, now became the conjugates in respect of the system C. 

 The image of the filament, no longer being in the aperture at p', 

 is not reproduced at f, the conjugate of p' in respect of the lens 

 FL' ; i.e., the beam at f is for all practical purposes now homo- 

 geneous. In the diagram, Fig. 8, which is drawn from the form 

 of the Gullstrand slit-lamp made by Messrs. Zeiss, also in the photo- 

 graph of the same apparatus in Fig. 9, K is the lamp switch (12) ; 

 a, a, a, are screws for centreing the lamp-filament about the optical 



Fig. 7. — Retroillumination. 



