Experimenting with Darkfield...

 My first attempts at Darkfield illumination (DF) a few weeks ago met with failure, in part due to misinformation on the internet (can that happen?) and lack of understanding on mine. I revisited the technique yesterday and much fell into place. Once I understood the stop had to obstruct ALL direct light going to the objective and the amount of direct light going into the objective is directly related to the NA. things became manageable.  There are methods available to calculate the exact size needed to obscure the direct light going into an objective but my stops were so easy to make that I just winged it. So let's get to work. I assembled a compass cutter, electricians tape, and a few old overheads from bygone presentations and a steel rule.

 The stops I was making are to be used with my 6.3X, 16X and 25X objectives  and I remembered reading about dimes and quarters being used so that became my starting point. Eventually, my 6.3X required something slightly smaller than a dime while the 25X needed a stop just larger than a quarter and the 16X was somewhere in the middle. 

The acetate circles were cut to fit my filter tray (32mm) and the electricians tape was added to just provide a black background when no slide was in place. Once you're finished just match the proper stop to it's companion objective, open up both the field  and aperture iris and be prepared to be amazed...especially, and perhaps only, if you have a suitable specimen on the stage. In my case I did, and I was totally blown away. I now understand what is meant when folks suggest the camera interpretation is nothing like the actual view. In my case, I saw what I might describe as diamonds thrown across a velvet sky were I the poetic sort. But I'm not so I'll just say the view was something out of this world. Perhaps a more suitable camera setup than mine can better represent what the eye really sees here but I doubt it. Here are a couple of images. Also you have to take into account the longer exposure required and the fact that these are motile diatoms...they move.

Pennate Diatoms in darkfield Illumination

DF image showing the large numbers of Daitoms in the FOV

 As the above image shows there were loads of these diatoms in my water sample, literally thousands within the area of the cover slip.