This is Fiji's fork of Michael Abramoff's "Biomedical Imaging in Java" (bij) ImageJ 1.x plugins. The original project website is here:

http://bij.isi.uu.nl/

This component contains the FlowJ, VolumeJ, SurfaceJ and Volume Reconstructor plugins only. For the library code upon which they are built, see:

https://github.com/fiji/bij

The original license for bij (as seen in source headers) was not open source:

Permission to use, copy, modify and distribute this version of this software or any parts
of it and its documentation or any parts of it ("the software"), for any purpose is
hereby granted, provided that the above copyright notice and this permission notice
appear intact in all copies of the software and that you do not sell the software,
or include the software in a commercial package.
The release of this software into the public domain does not imply any obligation
on the part of the author to release future versions into the public domain.
The author is free to make upgraded or improved versions of the software available
for a fee or commercially only.
Commercial licensing of the software is available by contacting the author.
THE SOFTWARE IS PROVIDED "AS IS" AND WITHOUT WARRANTY OF ANY KIND,
EXPRESS, IMPLIED OR OTHERWISE, INCLUDING WITHOUT LIMITATION, ANY
WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Fortunately, the author granted permission for the code to be relicensed as GPLv3:

From: Curtis Rueden
Date: Thu, May 7, 2020 at 4:41 PM
Subject: Relicensing BIJ with an open source license?
To: Michael Abramoff

Hi Michael,

I'm writing to ask if you would be willing to relicense your old "Biomedical Imaging in Java" (BIJ) project under a GPL-compatible open-source license -- either a permissive one such as BSD-2, or a copyleft one such as GPLv3.

We have distributed BIJ code with the Fiji project for many years (see > https://github.com/fiji/FlowJ and https://github.com/fiji/bij), but I do not know whether you ever gave permission for us to do this.

Therefore, I'm seeking your blessing now. If you are willing to allow the relicensing, we can keep distributing FlowJ et al. with Fiji. Otherwise, for legal reasons we would need to remove BIJ from the distribution.

How should we proceed?

Regards,
Curtis

From: Michael Abramoff
Date: Thu, May 7, 2020 at 4:45 PM
Subject: Re: [External] Relicensing BIJ with an open source license?
To: Curtis Rueden

Absolutely that is fine. Please go ahead.