I see that IanW has given a reference to a short discussion elsewhere on the forum.
The original articles by Strange are;
1. Strange, A., New Developments in Variable Asymmetrical Contrast Microscopy. MICROSCOPE 1989 (37) p355-376.
2. Strange, A., Variable Asymmetrical Contrast Microscopy. MICROSCOPE 1987 (35) p279-290.
Unfortunately, whilst I have photocopies of these, I do not find them all that useful, as they give very little practical detail of how to do this with your stand, if at all.
Mr B Wilkinson, in the UK took up this technique, and demonstrated it at meetings in the UK over a period of years, and it was well received. I persuaded him in 2011 to write up his methods, with full practical details, which appeared in the April issue of 'Balsam Post' the journal of the Postal Microscopical Society. This ten page article showed how to adapt the technique to a number of stands, namely using a Zeiss standard Abbe on an LCE, and phase 'turret' type condensers on Zeiss West and Reichert stands. Advice is given on cutting the discs, their size for named objectives, where to put them, and how to tweak the effect. There was also a follow-up article by Mr Wilkinson, and another by Mr Chaplin showing how he fitted his Biolam (Russian LOMO) stand for this.
Unfortunately, the PMS website (https://postalmicroscopicalsociety.github.io/index.html
) does not include the article text, only larger, colour versions of photographs used in the articles. Balsam Post is issued to members 4 times a year, and is in an A5 printed form, some 48 pages per issue.
I do have some back-copies of the issue containing the original article by Mr Wilkinson, and the Society can supply these, at a small cost. For overseas (ie. non UK) I may be willing to send a pdf of the article.
I am also willing to answer queries relating to this technique, as, like Mr Wilkinson, I feel that every microscopist should have it at their disposal. Personally, I have set it up on Zeiss (West), Reichert, CTS 2000, AO Series 10 and Nikon Optiphot stands with some success.
The articles mentioned above in Balsam Post are;
Strange's Variable Amplitude Contrast, Balsam Post 91.12, Brian Wilkinson (2011)
VAC Applications in Pond Life Microscopy, Balsam Post 100.22, B Wilkinson & J Wright (2013)
VAC 2 for Beginners, Balsam Post 102.34, Mick Chaplin (2014)
Lastly, to avoid folk thinking they are immediately going to get the effect that Spike Walker has in his shot of sponge spicules show below....he is a using a bi-polar filter.
I would suggest getting to grips with 'standard' VAC first.