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I have filled these many times.   Remove the four screws that hold the bubble chamber in place.  You will see a fill screw there at the bottom.  Remove it, and be sure to note and keep the steel ball which seals it.   Filling can be with clear odorless kerosene, but if you can obtain ChemSil D-1.5 silicone fluid this matches what was originally placed in these bubble chambers.  A fine needle syringe will do as a filler.  The problem though is that the seals may have failed causing the original silicone fluid to leak out and this means that you have to take this bubble chamber apart, and reseal it.  It is a very tricky job.   Original seals required shellac flakes melted by heating.  But I have discovered that the glass lenses have a distinct bevel and there are O rings that will fit into this groove and the retaining rings then tightened down to seal the two lenses.  But the bellows is another problem.  If that leaks then it is even harder to fix, but can be done with the right O ring.   I could not find one the right size or thickness and had to carefully hand sand it down to the right specs to replace the original clear plastic one that was in the bubble chamber bellows that had hardened with age.  And there is another way too.  Just take out the original dry sealing ring and the metal retainer, and get a O ring that fits into the bubble chamber bellows hole and screw down the retaining ring tight.  But you must make sure that the bellows disk is centered so that it seals properly.  You will note a very slight recess where the bellows seats. If off that recess the bellows will be damaged beyond repair if the retaining ring is tightened down.  But to to do this work on the bubble chamber you need to have the right tools to take it apart and put it back together.   There is also a problems with the fact that these have quite a bit of radium paint.  And this paint is extremely radioactive, much more so than what is placed on radium painted watches.   So you must be very careful not to contaminate yourself or where you are working.   I usually scrape this off after I wet it down with water to avoid radium dust and put the scrapings in can that I retain it till I find a way to legally get rid of it. (You just can't put it in the trash or dump it down the drain).   I then repaint the areas with modern bright green europium phosphorescent paint which works very well, as the original radioactive radium paint has long since burned out the sulphide, yet is still just as hot and dangerous as it was 70 years ago.  If you have any questions contact me directly at steve_schoner@yahoo.com.   And if you wish I can restore your bubble chamber at the cost of my labor and the items I put into it to make it right.