DIAPHRAGM-LAMP 



39 



Fig. 7, in which an iUuminated sectional area of the relucent lens- 

 tissue is chosen as the visual background. Such a small and 

 intense beam may be employed to yield other equally valuable 

 illumination-effects, but before these are discussed it is better, 

 at this stage, to describe the diaphragm-lamp and its principle 

 of production of a small beam of light of any desired shape and 

 size in cross-section. 



A point-source of light and a condensing lens will yield, in the 

 2:)osition conjugate to the point-source, a focal beam of small 

 diameter ; but the size and 

 shajDc of the point-source 

 cannot be varied at will so 

 as to alter the size and 

 shape of the beam at the 

 conjugate focus. If, how- 

 ever, the light from the 

 point-source (s, Fig. 8) is 

 collected and focused into 

 a small diaphragm-aper- 

 ture, p, of variable size and 

 shape, then this aperture r 

 may be used as a source 

 from which to form, by a 

 focusing lens, FL, a beam 

 whose size and shape can be 

 varied in accordance w'ith 

 the size and shape of the diaphragm-aperture ; the beam at f, the 

 conjugate focus of p, will have the shape of the aperture p, but be 

 of smaller cross-section, in conformity with the usual laws relating 

 to conjugate foci ; the shorter the focus of the lens, FL, the 

 smaller the diameter of the beam at f in relation to a given diameter 

 for the aperture p. In the Gullstrand slit-lamp the aperture 

 p is in the form of a vertically set slit whose jaws may be opened or 

 shut so as to provide a slit-aperture whose width can be varied at 

 will ; the source of light, s, is a vertically disposed condensed 

 spiral filament. In the original form of the lamp an image of the 

 filament was focused into the aperture p, i.e., s and p were con- 



FiG. 6. — Retroillumination. 



