RELIEF STAINING 123 



J-in. objective, after washing and before permanently 

 mounting, in order to see whether they are satisfactory. 

 If the film is on a slide, a drop of water is put on and 

 covered with a cover-glass ; if on a cover-glass, this is 

 mounted in a drop of water on a slide. If satisfactory, 

 the preparation can be dried, and mounted in balsam ; or 

 if not stained sufficiently or if stained too deeply, it can be 

 stained again, or further decolorised, as the case may be. 



Another process for demonstrating the presence of 

 organisms in films is by the method of " relief staining " 

 in which the organisms are left unstained on a coloured 

 background. This may be done by the Indian-ink method 

 (see " Syphilis ") or by Benian's Congo method. 1 For 

 this a small drop of a 2 per cent, solution of Congo red 

 in distilled water is placed on a slide and a very small 

 quantity of the bacterial culture, or of exudate, is rubbed 

 up with it with a platinum wire ; the drop is then spread 

 out into a tolerably thick film, either with the wire or 

 with another slide. The film is allowed to dry in the air 

 and is then washed with a 1 per cent, solution of hydro- 

 chloric acid in absolute alcohol and dried in the air, 

 preferably not blotted. The background is an opaque 

 blue and the organisms appear white. Broth and saline 

 solutions do not form a satisfactory mixture with the 

 Congo red and the organisms should be centrifuged out 

 of them. Serous exudates mix readily and evenly. Care 

 must be taken that the distilled water used in making 

 the Congo red solution contains no organisms. 



Treatment of Sections for Staining and Mounting 



(a) Frozen sections. If preserved in spirit they should 

 be rinsed in distilled water or in fresh alcohol before 

 staining, according as the staining solution is an aqueous 





Brit. Med, Journ., 1916, vol. ii, p. 722. 



