166 BACTERIOLOGY. 



Opening the dark-back, the plate may be turned out on to the 

 palm of the hand. The film side is then uppermost, and the plate is 

 to be transferred in the same position to a tray, and covered with 

 water. This is to soak the film and obtain an equal action of 

 the developer ; or the solution of fresh pyro may be poured on to 

 the plate without previous soaking, if the flow is uniform, and the 

 formation of bubbles avoided. In the first case the water is run off 

 and the pyro allowed to flow evenly over the plate. To protect the 

 plate from prolonged exposure to the ruby light, a second tray may 

 be inverted over it, or the developing tray covered with a piece 

 of card-board. Gently rock the tray for a minute or so, then 

 to a few drops of ammonia in a measuring glass add the pyro 

 from the developing tray, and pour the mixture back again 

 over the plate. After again gently rocking the tray for a few 

 minutes, more ammonia is added by drops in the same way. 

 If the exposure has been properly timed, and the time necessary 

 must be ascertained by trial for each preparation, the image will 

 gradually begin to appear, and the action must be allowed to 

 continue until sufficient density has been obtained. To determine 

 this requires some experience. It is generally recommended to take 

 the plate out of the tray and hold it for a moment, film-side towards 

 the operator, in front of the ruby light. Though the plate is not 

 nearly so sensitive when the image has commenced to develop, and 

 there is, therefore, not the same danger of fogging, a safer plan is 

 to occasionally turn the plate film downwards in the tray, and when 

 the image appears on the back the development will be found to be 

 completed. 



With such a preparation as tubercle bacilli in sputum it is 

 not easy to trace the gradual formation of the image, and hence the 

 advantage of commencing with a well-marked object such as the 

 blow- fly's tongue. It is then easy to watch the gradual progress of 

 the image. The bright parts or high-lights appear first, then 

 gradually the half-tones, or less brightly-lighted parts, and lastly 

 every shade except the deepest shadows is represented. When, 

 however, all action seems to have ceased, we must still wait until 

 we have judged, in the manner already described, that the density 

 is sufficient. This being determined, we pour off the developing 

 solution and thoroughly wash the plate with water. It is then 

 ready to be placed in dish No. 1, containing " hypo," and here it must 

 be left for some minutes after all appearance of creaminess has 

 disappeared from the back. White light may now be admitted, the 

 plate removed from the hypo and thoroughly washed under the tap, 



