WITH THE MICROSCOPE. 267 



free from any particles likely to be carried on to the plate by the 

 stream (the finger is commonly passed over these parts to clear them 

 away), pour the collodion with a steady flow on to the plate, a little 

 nearer to the left hand than its centre ; while flowing, depress the 

 lower and upper left corners, gradually, to bring the collodion fairly 

 to their edges, at the same time that the pool is being increased by 

 pouring, and then lower the plate to flow the fluid to the right further 

 corner, and pour off at the lower one into the bottle, resting the sides of 

 the angle on the lip, and rocking it to keep the plate slightly inclined ; 

 drag off, as it were, the lower part against the neck of the bottle, 

 close it, and hold the plate horizontally by the pneumatic holder for 

 10 seconds to half a minute, or even more, according to the setting 

 quality of the collodion ; if this be slow, it will be better to rest the 

 holder on some flat place or shelf, that the warmth of the hand may not 

 cause unequal evaporation. Once seeing this operation well done is 

 better than a lengthy description. Detach the plate from the holder 

 and place it on the fluted glass or silver wire dipper, to be plunged at 

 one gradual stroke into the nitrate bath. Here it is allowed to remain 

 for one minute, then raised and lowered several times so as to wash 

 the surface well, and permitted to remain in the bath for one or two 

 minutes longer, when the dipper with plate is to be steadily withdrawn ; 

 the plate removed, and rested by its lower edge on a pad of clean 

 blotting paper, the dipper returned to the bath, and the back of th.e 

 plate wiped with a pledget of clean rag> being gently steadied by the top 

 corners between the thumb and fingers, of the left hand, which must be 

 dry and clean. Open the back of the plate frame, pi. LIV, fig. 331, 

 and place with the right hand the plate into the frame, which should 

 be dry and free of dust, face downwards, close the back, cover the 

 frame with a large piece of black calico, carry its lower edge down to 

 the apparatus, and rest it against the wall or table. Re-adjust the 

 prism, remove the focussing screen, having glanced at the image on it, 

 set the covered card against the achromatic condenser, pass the slide 

 holder under the focussing cloth, into the position of the greyed 

 screen ; //// carefully the shutter of the frame, the hands being under 

 the cloth : let all remain for a moment or two that vibrations may 

 cease, snatch away the card without shaking, and replace it quickly, 

 allowing a period from half to twenty-five or thirty-five seconds for the 

 image to be impresssed ; the time must be learnt by practice ; close 

 the shutter gently, withdraw and replace the frame in the cloth, pass 

 the focussing screen into its place, again snatch away the card and 

 observe the image, then cover the prism and return with the slide 

 holder to the dark room, and proceed to develope the picture. The 

 reason for the re-observation of the image is, to see if the object and 



