410 E. HANBURY HANKIN. 



are likely to be permanent. It is well known that, cseteris 

 paribus, bacteria that have been stained for six hours are more 

 likely to retain their colour than those that have been in the 

 staining solution for only a few minutes. Is it then likely that 

 these methods will be permanent, where the whole process 

 takes only a few minutes, and the bacteria are only exposed to 

 the action of the dye for a few seconds ? Though the only 

 way to settle this question is to observe how long sections thus 

 stained will last, I thought that some indication of their 

 permanency or otherwise might be obtained by exposing them 

 to the action of sunlight. To do tliis I fixed up several sections 

 in a window with a southern aspect on sunshiny days and 

 observed the effect of the light at intervals. The results 

 obtained were very different in different cases, but seemed to 

 show that the permanency under these conditions depended 

 rather on the amount of colour that had been washed out of 

 the section after staining, than on the time that the section 

 had been left in the staining solution. 



A section that had been stained in gentian violet for twenty- 

 four hours in the ordinary manner was completely bleached by 

 sunlight in three hours. A section stained by the eosin and 

 methyl blue method, and in which all blue colour had been 

 washed out of everything except the bacteria, faded in the 

 same time, while other sections stained by the same method, 

 but in which a trace of colour had been left in the nuclei, stood a 

 whole day's sunlight witiiout much change, so far as the 

 micro-organisms were concerned. Another section which had 

 been stained in this method for histological purposes when 

 exposed to six hours' sunlight lost the colour previously 

 adhering to the nuclei, but left the bacteria present still 

 strongly dyed. It is thus clear that the fastness to light is due 

 to the quantity of dye present in any stained part of the section ; 

 and I employed this fact in differentiating out certain bacteria by 

 means of light as follows. I had some sections of the liver of 

 a mouse that had died of some form of septicaemia investigated 

 by Dr. Klein. The sections when stained with Spiller's purple 

 and eosin showed here and there dark blue apparently homo- 



