HISTOLOGICAL SUBJECTS. 211 



Collodion plates with the same objectives require but one cell if 

 the specimen is very thin and delicately coloured, and the same 

 exposure — about one-third to three-quarters of a second. Powers 

 from one-fifth of an inch and upward do not require any blue cell, 

 unless there is a difference between the chemical and visual foci. 

 For collodion plates one and a-half seconds are sufficient for the 

 exposure with ordinary carmine-stained sections ; then the time 

 lengthens rapidly as the powers increase. 



The advantages of the short exposure are, that the middle 

 tones do not have time to be reproduced, only the marked lights 

 and shadows are impressed, and there is much less danger of 

 jarring the apparatus than when the exposure occupies several 

 minutes. In a city where vehicles are constantly passing, it is 

 impossible to get a perfectly quiet five minutes, as even what is 

 not a very perceptible jar will mar the clearness of the outlines- 

 Moreover, in the case of the short exposure, less time is allowed 

 for the occurrence of unforeseen accidents. 



Dry plates must be handled, as usual, under ruby light alone- 

 Ferrous oxalate is the best developer, giving perfectly clear, white 

 shadows ; eight parts of a saturated solution of potassic oxalate, 

 acidulated with oxalic acid, to one part of a saturated solution of 

 ferrous sul]ihate, acidulated with sulphuric acid, is tlie usual for- 

 mula. It is advantageous to add two or three drops of a saturated 

 potassium-bromide solution. Or the develo]Mnent may be com- 

 menced in a normal developer, and the.i transferred to one con- 

 taining bromide. Should the image begin to appear in a normal 

 developer under fifteen seconds, the plate has been more than 

 fully exposed, and very decidedly too much so for microscopical 

 purposes. It might, perhaps, be saved by promptly adding bro- 

 mide ; probably, another plate must be taken. Develop until the 

 back is quite grey in the shadows. With the ferrous oxalate deve- 

 loper and a properly e\i)Osed plate, there is scarcely any danger of 

 over-development. If, after fixing, the negative appears a little too 

 lieavy in the fine details, returning it for an hour or more to a 

 strong hyposulpliite solution may restore it. If that docs not 

 bleach it sufficiently, wash and pour on and off the jjlate a very 

 weak solution of ])otassium iodide, watching carefully all the while 

 that the [jrocess of changing the reduced silver to an iodide does 



