Summary

With reference to Jbeale's original idea:
I just changed the internal barrel id up to 25.4mm which then slides nicely over the microscope tube. I've added the STL file but it's easy enough to start with jbeale's original openscad file. This particular budget microscope only has a sub-stage plano/concave mirror so I've used a torch to illuminate the (blood smear) slide. I think you will get improved pictures in natural daylight or by using proper illumination. Note I've removed the original pi camera lens and I'm not using the microscope's eye piece lens. Also Jbeale's idea is for camera v2. Camera v1 has some additional surface mounted chips so you just need carve a pocket into the mating face.

use
raspivid -k -o dummypreview
while focusing and to take a picture:
raspistill -o .jpg

There are tons of settings to play with on the Pi Camera
and you could even do some time lapse filming.

https://www.raspberrypi.org/documentation/raspbian/applications/camera.md

Revision 0:

I've added a PI zero mounting plate. Use 4 standoff's to connect to the pi zero. This also keeps the ribbon connector tidy. The mounting plate rebate is now designed for v1 and v2 Pi cameras.

An objective sleeve that fits the larger dia 25.4mm barrel so you can mount the objective as per JBeales's telephoto lens. Slide the sleeve into barrel and clamp up the objective when the image comes into sharp focus.

I've supplied the openscad files because your objective may have different dimensions.

Learn more about microscopes: http://www.microscopy-uk.org.uk/