The design system is also know as a parallel microscope; having the two separate channels operating parallel to each other. This design feature guarantees that the convergence of light projected towards the left and right channels from the central objective coincides with the focal point in the specimen plane. Greenough utilizes inclined twin body tubes to produce a zero effect.

The objective lens is, at its simplest, a very high powered magnifying glass, a lens with a very short central length. This is brought very close to the specimen being examined so that the light from the sample comes to a focus about 160 mm inside the microscope tube. This creates an enlarged image of the subject.

Visual acuteness: The first step is to record visual acuity. Patients who require remedial lenses should wear them during trying. Visual acuity in each eye is tested as the opposite eye is covered.