A trinocular is virtually the same as a binocular microscope but with a third eyepiece tube. No - not for people with 3 eyes, but for people who wish to attach a camera to the third eyepiece and be able to photograph or video what they can see through the two eyepieces. So, only consider a trinocular microscope if photography is a critical desire to do. One of the purpose is to allow a second viewer access to a specimen at the same time as the person who is mainly using the microscope. This means that an instructor can, for example, look at what a student is seeing to answer a question or to check the accuracy of the student's observations. In addition, the instructor can invite a student to share his or her view of an interesting specimen or when modelling how to adjust the focus.
originally found here:- http://www.lab360.co.in/microscopes-telescopes.htm
