80 



MICROSCOPY OF THE LIVING EYE 



slit-beam, but is visible when a round beam of small diameter is 

 used owing to contrast with the surrounding unilluminated area 

 (Fig. 47). Naturally this visibility will vary with the intensity of 

 the initial source of light, being greater when an arc-lamp slit- 

 lamp is used. The observer who wishes to judge between normal 

 and pathological relucency of the aqueous fluid must accustom 

 himself to the particular apparatus he uses. Certain careful 



Fig. 47. — Cylindrical beam traversing the aqueous fluid 



between cornea and lens. 



adjustments are needed in order to avoid possible fallacies (11 and 

 18). 



When discrete particles are present in the aqueous fluid their 

 movement discloses the convection-currents in the fluid, beinw 

 downwards in the anterior part where the fluid nearer the surface 

 is cooler and upwards in the deeper posterior regions. Fig. 46, A, 

 shows pigmented and unpigmented cellular and amorphous pre- 

 cipitates on the anterior capsule of the lens, and, on the pupillary 

 border of the iris, two very characteristic nodular excrescences, 



