THE FOCUSING LENS 



43 



be pointed out here that the part of the beam used is the foeal 

 region (f, Figs. 8 and 11), and the learner must accustom himself 

 to judge the accurate focusing so as not to be working with the 

 ])refocal or postfocal region of the beam. The lens FL is usually 

 of either 10-0 cm. or 7-0 cm. focal length ; the shorter the focus 

 of the lens the nearer must it be placed to the eye under observa- 

 tion, and the more abrupt, and more easily appreciated, will be 

 the transition to the focal region of the beam from either the 



A B c D £ 



Fig. 10. — Common errors in focusing, setting and centreing of the 

 filament image on the diaphragm of the focusing lens. A — image 

 out of focus, the lamp being too far from collecting-lens ; B — image 

 out of focus, the lamp being too near collecting-lens ; C — image 

 in focus but incorrectly set so that when the vertical Arruga screw 

 is used the diaphragm-aperture runs off the image ; D — image in 

 focus, but incorrectly centred ; E — image correctly focused, set 

 and centred. 



prefocal or post focal regions (Fig. 11). The height -adjustment of 

 tlie illuminating beam is provided grossly by the milled-head N 

 (Fig. 9) on the upright column, and finally and finely by the 

 screw Y acting on the focusing lens which, when the filament- 

 image is correctly focused, set and centred (as in Fig. 10, E), is 

 readily moved up and down in the vertical plane of the 

 diverging beam which has emerged from the body of the slit- 

 lamp. Swivelling of the lamp is provided at E (Fig. 9) ; G is 

 simply a diaphragm-tube to cut off redundant light, and one end 

 of it is sometimes fitted with a rotatory screen (not shown in 



