Biologists, Clinical Researchers, Druggists, Chemical engineers etc often analyze image obtained from microscopes etc. This is a specialized image processing and requires specialized software. Often, these software are not reachable for independent researchers sometimes. If you are biologist or a hobbyist just trying to understand microscopic image processing, then you could try these open source software tools. They may not be as capable as the proprietary software but would be a great way to start. 

CellProfiler

CellProfiler is a free and open source software that helps biologists analyze cell/non cell images without much knowledge on computer or programming. Lots of tutorials for you to start!

Tissue Sample

Fiji

Fiji is an plug-in oriented image processing package. It is based on ImageJ but with Java, Java 3D and a lot of plug-ins organized into a coherent menu structure. The main focus of Fiji is to assist research in life sciences, helps review images. It is easy to install and has an automatic update function, bundles a lot of plug-ins and offers comprehensive documentation.

ImageJ

ImageJ is a Java based image processing software that run as an applet as well as a downloadable app. It can display, edit, analyze, process, save and print 8-bit, 16-bit and 32-bit images. It can read many image formats including TIFF, GIF, JPEG, BMP, DICOM, FITS and "raw". It supports "stacks", a series of images that share a single window. It is multithreaded, so time-consuming operations such as image file reading can be performed in parallel with other operations.

Endrov

Endrov is a multi-purpose image processing tool. It can analyze images from Microscope too.